Evaluate the following integral by interpeting it in terms of areas:

The definite integral of f(x) from 1 to 0

f(x) = sqrt(1 - x^2)dx =(1/4)3.14x^2= (1/4)3.14(1)^2 = 3.14/4

This is an example that was given in my text book. I don't undersatnd the second step (that part in bold type). Can someone please exlpain why the anti-derivative of sqrt(1 - x^2)dx is (1/4)3.14x^2?