Integration - Substitution (1 Viewer)

[micuzzo]

I have a qyick question... when doing integration with substitution, say S x(x^2 - 9)^5 , u=x^2 - 9 (where S is the integral sign) is it correct to sub for dx instead of du... so:

du/dx = 2x so dx = du/2x

thus S x u^5 du/2x
= 1/2 S u^6/6 etc... or must i do it like this;

du/dx=2x so du=2xdx

and S x(x^2-9)^5 dx = 1/2 S (x^2 - 9)^5 2xdx

..................................= 1/2 S u du
..................................=1/2 U^6/6.... etc...


??

 

[Dumbledore]

either will work

 

[AnandDNA]

nah u cant mix up variables. keep dx's and x's away from du's and u's. hsc markers will penalise ou for doing that, not sure whether your teacher is that strict

 

[gurmies]

nah u cant mix up variables. keep dx's and x's away from du's and u's. hsc markers will penalise ou for doing that, not sure whether your teacher is that strict

Yeah, pretty sure my friend got penalised for that. Seems a little unfair, the answer is the same in the end.

 

[micuzzo]

Yeah, pretty sure my friend got penalised for that. Seems a little unfair, the answer is the same in the end.

ok i see...

thanks for every 1's help