# When would you use u substitution twice?

When we are reversing a differentiation that had the composition of three functions. Here is one example.

int sin^4(7x)cos(7x)dx

Let u=7x. This makes du = 7dx and our integral can be rewritten:

1/7 int sin^4ucosudu = 1/7int(sinu)^4cosudu

To avoid using u to mean two different things in one discussion, we'll use another variable (t, v, w are all popular choices)

Let w=sinu, so we have dw = cosudu and our integral becomes:

1/7intw^4dw

We the integrate and back-substitute:

1/7intw^4dw = 1/35 w^5 +C

 = 1/35 sin^5u +C

 = 1/35 sin^5 7x +C

If we check the answer by differentiating, we'll use the twice.

d/dx((sin(7x))^5) = 5(sin(7x))^4*d/dx(sin(7x))

 = 5(sin(7x))^4*cos(7x)d/dx(7x)

 = 5(sin(7x))^4*cos(7x)*7

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