# Am I missing something obvious?Watch

This discussion is closed.
#1
I've been doing an integration based problem and the worked solution to it evaluates the integral with respect to x of
x/(3-0.5x^2)^2
as
1/(3-0.5x^2)

which isn't something I'm very familiar with. Can anyone see how this is arrived at? I think I could be missing something simple, but the more I think about it the less likely I am to see it!

(I can integrate using a substitution, but that gets ln functions etc which isn't wanted.)

Thanks for any help.
0
14 years ago
#2
Its function and derivative.

You know what the answer is so I'll show you with that.

If you take your function to be
y=(3-0.5x^2)^-1
then derivative by chain rule (i've used * as multiply)
dy/dx=-1*-(1/2)*2x(3-0.5x^2)^-2
which is x/(3-0.5x^2)^2

when you see an x at the top of a fraction and there is an x^2 function at the bottom, when finding the integral always try function and derivative first.
0
#3
Yes - just look at the fraction and think about what its inegral is. That's the problem with learning standard integrals (and doing STEP questions) - you expect that it's a clever substitution or something.

Thanks.
0
14 years ago
#4
I've been there - I'm always with the missing of the obvious. Good luck with STEP - you have a lot of courage!!
0
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