What is the integral of xcos(kx) ?

The integral of xcos(kx) is 1/(k^2)(kxsin(kx)+cos(kx))+C

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integral of xcos(kx)

\int x cos (k x) d x

\frac{1}{k ^{2}} (k x sin (k x) + cos (k x)) + C

Solution steps

\int x cos (k x) d x

Apply u-substitution

= \int \frac{u cos ( u )}{k ^{2}} d u

Take the constant out:

\int a \cdot f (x) d x = a \cdot \int f (x) d x

= \frac{1}{k ^{2}} \cdot \int u cos (u) d u

Apply Integration By Parts

= \frac{1}{k ^{2}} (u sin (u) - \int sin (u) d u)

\int sin (u) d u = - cos (u)

= \frac{1}{k ^{2}} (u sin (u) - (- cos (u)))

Substitute back

u = k x

= \frac{1}{k ^{2}} (k x sin (k x) - (- cos (k x)))

Simplify

= \frac{1}{k ^{2}} (k x sin (k x) + cos (k x))

Add a constant to the solution

= \frac{1}{k ^{2}} (k x sin (k x) + cos (k x)) + C

What is the integral of xcos(kx) ?
The integral of xcos(kx) is 1/(k^2)(kxsin(kx)+cos(kx))+C