Integrate ∫ 3x cos x dx Step by Step

Step by step

The question

Question: find the integral of 3x cos x.

∫ 3x cos x dx =

1 Choose u and dv

Choose u = 3x: its derivative is simpler, just a number.

dv is the rest: cos x dx, which we can integrate.

u = 3x, dv = cos x dx

2 Find du and v

du: the derivative of u, times dx.

v: the integral of dv.

cos gives sin.

du = 3 dx, v = sin x

3 Use the rule

The rule: the integral of u dv is u v, minus the integral of v du.

It comes from the product rule, backwards.

∫ u dv = uv − ∫ v du

u times v, minus the integral of v times du.

= 3x sin x − ∫ 3 sin x dx

4 The new integral

The new integral is easy: just a number times sin.

sin integrates to minus cos.

= 3x sin x − (−3 cos x)

Careful!

It is u v minus the integral of v du.

Not plus!

Careful: not uv + ∫ v du ✗

Open the bracket: a minus in front of a bracket changes the sign of every part inside.

Add C.

So the answer is 3x sin x + 3 cos x + C.

= 3x sin x + 3 cos x + C

5 Check

Let's check: take the derivative of our answer.

The product rule on each term: the first's slope times the second, plus the first times the second's slope.

derivative: 3 sin x + 3x cos x − 3 sin x

The other terms cancel, and we get 3x cos x: the question again.

Right!

= 3x cos x

Let's look back.

First: choose u and dv.

Then: find du and v.

Then: use the rule.

Then: the new integral.

Last: check.

Answer