Use integration methods to establish the formula A = π r^2 for the area of a disc ofradius r.

so the equation of the circle is x^2 +y^2 =r^2 . i will try and find the area of one quadrant using integration. so it will be ∫ r,0 y dx

so ∫ r,0 √(r^2 -x^2) dx so from here i am trying to integrate it but it is not working for me. i saw some suggested help saying you should substitute x for something but i do not know what it is and i can not see why you would do this either. i cant see why my normal method should not work.