P.O.I Question

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  • #1
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Homework Statement


The slope of the curve y = ax(cubed) + bx(squared) + c is -12 at its point of inflection (2,-11), find a,b,c.


Homework Equations


f ' (x) = 3ax(squared) + 2bx
f " (x) = 3ax + 2b


The Attempt at a Solution



f ' (x) = 3ax(squared) + 2bx
-12 = 3a2(squared) + 2b(2)
-12 = 12a + 4b
b = -3a - 3

-12 = 3a(2)(sqrd) + 2(-3a -3)
a = -1

b = 0

-11 = ax(cubed) + bx(sqrd) + c
c = -3
 

Answers and Replies

  • #2
2,063
2
That is correct.

Another way to do it, and probably the one "expected" from you, is to use the fact that at the point of inflection, f''(x)=0.
 
  • #3
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not exactly sure how to do that..

6ax + 2b = 0?
 
  • #4
2,063
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Yes, that's right. Therefore b = -6a. Solve for a.
 
  • #5
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so b = 6 not 0?
 
  • #6
2,063
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so b = 6 not 0?
No, no!!
b = -6a is one relation, and is b = -3a - 3 is another. Now solve for a. Sorry for the confusion.
 
  • #7
631
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ur solution in OP is wrong . Check out step 5.
 
  • #8
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which do you consider step 5?
 
  • #9
631
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-12 = 3a(2)(sqrd) + 2(-3a -3)(?)
a 2 is missing in (?)
 
  • #10
37
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ok so a = 0 now
 
  • #11
2,063
2
Sorry, my bad. As Sourabh has pointed out, your solution in the first post is in fact incorrect. a is not 0. What you would get from "step 5" is 0=0. That is because you got the relation bet. a and b from that very equation.

As I said earlier, use b = -6a and b = -3a -3 to solve for a.
 
  • #12
37
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-12=3ax(sqrd)+2(-6a)x
a=-1
b=6
c=-27
 
Last edited:
  • #13
2,063
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a=-1
Check again.

Once you get values for a, b and c, you can check if they are right by putting x=2 in the original equation and looking at the value you end up with.
 
  • #14
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ok im confused now.. ive found new values fro a(1),b(6) and c(5) but x doesnt = 2 when i enter it into the original equation.
 
Last edited:
  • #15
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i think there's again a calculation mistake in #12.
-12 = 3a2(sqrd) + 2(-6a)2
-12 = -12a
a=1.
 
  • #16
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ok i got that one are the other two right? b=6 and c=5?
 
  • #17
2,063
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b = -6 (b = -6a, remember?)
 
  • #18
631
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no. b=-6a=-6

-11 = 1*2(cubed) - 6*2(sqrd) +c
 
  • #19
37
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a=1, b=-6, c =5
 
  • #20
37
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is this correct?
 
  • #21
8
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You should get 3 equations:

[tex]12a + 2b = 0[/tex] (from 2nd derivative)
[tex]12a + 4b = -12[/tex] (from 1st derivative)
[tex]8a + 4b + c = -11[/tex] (from original equation)

Just solve for a,b,c from that info... I think you have all the right ideas, just getting wrong values.
 
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