Step by step
The question
Question: find the integral of 2x times √(x² + 1).
1 Choose u
The inside: x² + 1.
Call it u.
Its derivative is 2x, and 2x is outside.
A good sign: u′ is there, or nearly.
2 Find du
The power rule: du/dx = 2x.
Multiply both sides by dx: du = 2x dx.
A small change in u is 2x times a small change in x.
3 Write it with u
x² + 1 becomes u, and 2x dx becomes du.
A square root is the power one half.
4 Integrate
Integrate: add 1 to the power, divide by the new power.
One half plus 1 is three halves.
Dividing by three halves is the same as times 2/3.
5 Put x back
The question was in x, so the answer is in x too.
Put x² + 1 back in place of u.
So the answer is 2/3 × (x² + 1)^(3/2) + C.
6 Check
Let's check: take the derivative of our answer with the chain rule.
The power comes down and cancels the number below.
Then times the inside's slope, 2x.
That is 2x√(x² + 1): the question again.
Right!
Let's look back.
First: choose u.
Then: find du.
Then: write it with u.
Then: integrate.
Then: put x back.
Last: check.