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# C4 integration using substitution help watch

1. Hi, just wondering if someone could help me with an integration using substitution question (when you are given u=...). I know you work out du/dx and substitute this and u into the integration and cancel out dx, then work it out.. But I can't see how it works on some questions - like this one..

Use the substitution u= 3x - 1 to express integral of x(3x-1)^4dx as an integral of x

Thank you for any help
2. What is x in terms of u?

Can you solve it now?
3. sometimes the x will not cancel as in this case... instead you rewrite x in terms of u and go from there...
4. (Original post by b4nanapancakes)
Hi, just wondering if someone could help me with an integration using substitution question (when you are given u=...). I know you work out du/dx and substitute this and u into the integration and cancel out dx, then work it out.. But I can't see how it works on some questions - like this one..

Use the substitution u= 3x - 1 to express integral of x(3x-1)^4dx as an integral of x

Thank you for any help
Make x the subject of u = 3x -1 then sub that in along with u4 into your integral.
Then its just normal integration.

EDIT: Beaten to it.
5. (Original post by aznkid66)
What is x in terms of u?

Can you solve it now?
Still a bit stuck.. What about the dx? Do I replace the 3 in (u+1)/3 with du/dx ?

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6. (Original post by b4nanapancakes)
Still a bit stuck.. What about the dx? Do I replace the 3 in (u+1)/3 with du/dx ?

This was posted from The Student Room's iPhone/iPad App
You've got du/dx = 3?
Then dx = du/3
Divide the function in the integrand by 3.
Then integrate the function with respect to u.
7. (Original post by joostan)
You've got du/dx = 3?
Then dx = du/3
Then integrate the function with respect to u.
Ah ok! Great thank you for your help!

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8. (Original post by b4nanapancakes)
Ah ok! Great thank you for your help!

This was posted from The Student Room's iPhone/iPad App
you're welcome

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