# Homework Help: MATLAB - floating point numbers

1. Jul 14, 2011

### GreenPrint

1. The problem statement, all variables and given/known data

Consider the following two equations:

x^2 + y^2 = 42
x + 3y + 2y^2 = 6

Define a symbolic equation for each, and solve it by using MATLAB's symbolic capability. Could you solve these equations by using matrices? Try this problem twice, once using only integers in your equation definitions and once using floating-point numbers (those with decimal points). how do your results vary? Check the workspace window to determine weather the results are still symbolic.

2. Relevant equations

3. The attempt at a solution

I think that I may be doing something wrong because when I wrote out my code and ran it I got the same exact results when I tried using integers and when I included decimal points. I was hoping someone could explain to me what I'm doing wrong because I guess I'm suppose to get different results.

Code (Text):
>> disp('No you could not solve this problem usign matrices.')
one=sym('x^2+y^2-42');
two=sym('x+3*y+2*y^2-6');
[x,y]=solve(one,two)
one=sym('x^2.0+y^2.0-42.0');
two=sym('x+3.0*y+2.0*y^2.0-6.0');
[x,y]=solve(one,two)
No you could not solve this problem usign matrices.

x =

-6.2161908711674029137999766546085
6.4782037201238076694174751205659
6.3321946913754454971273459117746
-5.594207540331850252744844377732

y =

1.8327495882457713513416277757555
-0.18131894709064188368251606877471
-1.3796051574695662000556283784362
-3.2718254836855632676034833285446

x =

-6.2161908711674029137999766546085
6.4782037201238076694174751205659
6.3321946913754454971273459117746
-5.594207540331850252744844377732

y =

1.8327495882457713513416277757555
-0.18131894709064188368251606877471
-1.3796051574695662000556283784362
-3.2718254836855632676034833285446

2. Jul 15, 2011

### viscousflow

you can change the accuracy of symbolic data with digits() otherwise this is its default output.

3. Jul 15, 2011

### GreenPrint

Interesting. Are textbook hasn't covered the digits function yet. I'll go look up how to use it.

4. Jul 15, 2011

### GreenPrint

I'm confused as to how to use the digits function and incorporating it into this problem.

5. Jul 15, 2011

### MATLABdude

I'm not really familiar with the Symbolic Math Toolbox, but a quick search at the Mathworks' MATLAB documentation page brings up the following:
http://www.mathworks.com/help/toolbox/symbolic/f1-5556.html [Broken]

If you don't already know, you can also access abridged documentation from the MATLAB command prompt by typing in help <function>. For instance:
>> help digits

Last edited by a moderator: May 5, 2017
6. Jul 15, 2011

### GreenPrint

It's my understand that digits(x) just changes how many digits are displayed in results to x. At least that's what function seems to do and what the help document says. I thought i was suppose to get different results when I did 5*x and 5.0*x or something. I'm pretty sure I don't need the digits function as this is the first time I have come across it. Hmm...

7. Jul 15, 2011

### GreenPrint

I was expecting something along the lines of

Code (Text):
>> solve('5.0*x^2.0+6.0*x-7.0')
solve('5*x^2+6*x-7')

ans =

0.72665
-1.9266

ans =

(2*11^(1/2))/5 - 3/5
- (2*11^(1/2))/5 - 3/5
were the results actually varied

8. Jul 15, 2011

### MATLABdude

Odd, the lab computer here seems to produce the correct result:

Code (Text):

x =

[ 33/4-1/4*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-1/4*6^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)-301*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+9*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)-2*(-3/4+1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+1/12*6^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)-301*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+9*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2))^2]
[ 33/4-1/4*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+1/4*6^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)-301*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+9*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)-2*(-3/4+1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-1/12*6^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)-301*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+9*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2))^2]
[   33/4+1/4*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-1/4*(-(-330*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+6*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)+1806*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+54*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)-2*(-3/4-1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+1/12*(-(-330*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+6*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)+1806*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+54*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2))^2]
[   33/4+1/4*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+1/4*(-(-330*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+6*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)+1806*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+54*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)-2*(-3/4-1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-1/12*(-(-330*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+6*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)+1806*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+54*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2))^2]

y =

[ -3/4+1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+1/12*6^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)-301*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+9*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)]
[ -3/4+1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-1/12*6^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)-301*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+9*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)]
[  -3/4-1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+1/12*(-(-330*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+6*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)+1806*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+54*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)]
[  -3/4-1/12*3^(1/2)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)-1/12*(-(-330*(2033+18*i*71413^(1/2))^(1/3)*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+6*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)*(2033+18*i*71413^(1/2))^(2/3)+1806*((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2)+54*3^(1/2)*(2033+18*i*71413^(1/2))^(1/3))/(2033+18*i*71413^(1/2))^(1/3)/((55*(2033+18*i*71413^(1/2))^(1/3)+2*(2033+18*i*71413^(1/2))^(2/3)+602)/(2033+18*i*71413^(1/2))^(1/3))^(1/2))^(1/2)]

EDIT: Also, have you tried using the x=syms('x') and y=syms('y') approach? I should add that we have an ancient copy of MATLAB here, R13 (v6.5)

9. Jul 15, 2011

### GreenPrint

I'm not sure what exactly you mean by incoperating x=syms('x') and y=syms('y') approach???

I thought syms was for creating symbolic representations of variables and you just did
syms x y
I'm not exactly sure what x=syms('x') and y=syms('y') even does

Code (Text):
>> x=syms('x')
??? Error using ==> syms
Too many output arguments.
I don't think I follow.

also I'm using MATLAB 7.12.0(R2011a)

10. Jul 15, 2011

### MATLABdude

Ah, syntax changed. If you read the link in my previous post, you'll see that the proper syntax is:

Code (Text):

syms x y
one=x^2+y^2-42
two=x+3*y+2*y^2-6
[x,y]=solve(one, two)

11. Jul 15, 2011

### GreenPrint

Code (Text):
>> syms x y
one=x^2+y^2-42
two=x+3*y+2*y^2-6
[x,y]=solve(one, two)

one =

x^2 + y^2 - 42

two =

2*y^2 + 3*y + x - 6

x =

-6.2162
6.4782
6.3322
-5.5942

y =

1.8327
-0.18132
-1.3796
-3.2718
=(

It only went out to 5 digits from when I was experimenting with the digit function I did digit(5)

12. Jul 15, 2011

### MATLABdude

In this matter, MATLABdude is a MATLABdud.

Have you tried the CS solution? Exit MATLAB and then start it up again?

13. Jul 15, 2011

### GreenPrint

I don't know what you mean by CS, but I tried exiting and restarting MATLAB and the same results occurred.

14. Jul 19, 2011

### GreenPrint

Does anyone have any other solutions that might work?

15. Jul 20, 2011

### GreenPrint

hmm... I find it odd that it worked properly in a older version of matlab >_>

16. Jul 20, 2011

### GreenPrint

could someone please confirm with a newer version of matlab that there getting the same results?

17. Jul 20, 2011

### GreenPrint

still stuck

18. Jul 22, 2011

### GreenPrint

Can someone with a newer version of matlab please run my code and let me know what results they get?

19. Jul 22, 2011

### The Electrician

Using Mathematica, I see that the problem amounts to finding the intersections between a circle and a parabola. There are only 4 intersections, so I would expect 4 solutions.

See the attachment:

#### Attached Files:

• ###### Prob1.png
File size:
7.2 KB
Views:
120
20. Jul 22, 2011

### GreenPrint

ya i was expecting to get an exact result though as the other person and not a rounded answer though

21. Jul 23, 2011

### GreenPrint

I'm still trying to get MATLAB to give me the exact results as the other person did but I can't seem to get MATLAB to do so...

I'm trying to get MATLAB to give me the exact results by trying to get around this in some other way... I have noticed one thing

x^2 + y^2 = 42
x + 3y + 2y^2 = 6

the second equation is linear
x + 3y + 2y^2 = 6
x=6-3y-2y^2
and i can substitute into the first equation
x^2 + y^2 = 42
x=6-3y-2y^2

(6-3y-2y^2)^2+y^2=42
but
Code (Text):
solve('(-3*y + -2*y^2 + 6 )^2 + y^2 = 42','y')

ans =

1.8327495882457713513416277757555
-0.18131894709064188368251606877471
-1.3796051574695662000556283784362
-3.2718254836855632676034833285446
no cigar... any suggestions would be greatly appreciated