For a circle with radius r, if the radius increases X and the area of the circle increases y, then the functional relationship between Y and X?

For a circle with radius r, if the radius increases X and the area of the circle increases y, then the functional relationship between Y and X?

The original area is π R & # 178;
So y = π (R + x) &# 178; - π R & # 178;
That is y = π X & # 178; - 2 π Rx