
#1
Aug810, 12:54 PM


#2
Aug810, 01:04 PM

P: 851

[tex]\frac{8}{9(0.5 + 0.441i)(2*0.441i)}[/tex] = [tex]\frac{8}{3.50066 3.969 i}[/tex]
Now you can multiply the top and bottom by 3.50066 + 3.969i ( which is the complex conjugate of the denominator) . When you simpify you will arrive at the answer. 



#3
Aug810, 01:24 PM

P: 38

How did you simplify the bottom line down to 3.50066  3.969i ?
What do you do with the 9? I find it confusing because its (...+...)(...x...) not ++ or xx My basic maths has left me lol. Thanks 



#4
Aug810, 01:46 PM

P: 851

Laplace transform into partial fractions, then I'm stuck!9(a +bi) = 9a + 9bi (a+bi)(c+di) = a*c + (a*d)i + (b*c)i  b*d (2*ai) = (2*a)i (*) denotes multiplication. 



#5
Aug810, 04:09 PM

P: 38

I kinda get you, see if this is right:
8  9(0.5+0.441i)(2*0.441i) 8  (4.5+3.969i)(2*0.441i) 8  8 1.9845i + 7.938i +1.75i 8  8 + 7.7035i which equals!! 1 + 1.038 is that more or less correct do you think? 



#6
Aug810, 04:29 PM

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No, it's not correct. How did you go from




#7
Aug810, 04:31 PM

P: 38

4.5 * 2 = 8
4.5 * 0.441i = 1.9845i 3.969i * 2 = 7.938i 3.969i * 0.441i = 1.75i just used that 'foil' method 



#8
Aug810, 04:48 PM

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You can't FOIL because the second factor isn't a sum. Also, 4.5*2 isn't 8; it's 9.
Just out of curiosity, what's the goal of this problem? To find y(t)? I just ask because I find the method used kind of questionable. 



#9
Aug810, 05:06 PM

P: 38

oh yea, woops
The question is: obtain the process response Y(t) as deviation from its initial steady state condition y(0). Use Laplace transforms and expansion by partial fractions. Assume a unit step forcing function x(t) = u(t) The initial equation is 9y''(t) + 9y'(t) +4y(t) = 8x(t)  4 



#10
Aug810, 05:26 PM

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OK, the solutions you have took a lessthanideal approach. It'll work, but it relies on working with complex numbers more than is necessary. When you work with factors of i, it greatly increases your chance of making sign mistakes, so if you can avoid working with them, it's usually for the best. Also, the solutions evaluated the roots numerically too early. it's usually not a good idea to evaluate numerical expressions midway through the problem because you end up with decimals, like 3.969, that you have to copy from line to line and sometimes you drop a digit or transpose two of them, etc. It's just asking to make a mistake. Plus, like any time you plug numbers in too early, it obscures possible cancellations down the road.
Anyway, back to your problem... You made one other mistake. You did something like [tex]\frac{a}{x+iy} \rightarrow \frac{a}{x}+i\frac{a}{y}[/tex] which isn't correct. What you need to do is multiply the top and bottom by the complex conjugate of the denominator, so you get [tex]\frac{a}{x+iy} = \frac{a}{x+iy}\times\frac{xiy}{xiy} = \frac{a(xiy)}{x^2+y^2} = \frac{ax}{x^2+y^2}  i\frac{ay}{x^2+y^2}[/tex] 


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