Step by step
The question
Question: solve y′ = 4x.
y′ = 4x
1 The idea
y′ is only about x.
So y is a function whose derivative is 4x: an antiderivative.
y′ = 4x: y is a function whose derivative is 4x
2 Integrate both sides
Integrate both sides.
The left side, the integral of y′, is just y.
y = ∫ 4x dx
The power rule backwards: add 1 to the power, divide by the new power.
∫ xn dx = xn + 1n + 1
Each term by its rule, and one + C: the number we cannot see after a derivative.
y = 4 × x22 + C
Tidy the numbers: y = 2x² + C.
y = 2x2 + C
Careful!
Don't forget + C.
Every number C gives another solution.
Careful: not y = 2x2 ✗
The answer: y = 2x² + C.
C can be any number: one solution for each C.
y = 2x2 + C
3 Check
Let's check: the derivative of our answer (the C goes, its derivative is 0) is 4x.
The right side again.
Right!
y′ = 4x
Let's look back.
First: the idea.
Then: integrate both sides.
Last: check.
Answer
y = 2x2 + C