y = √(a² - x²)
dy/dx = d√(a² - x²)/d(a² - x²) * d(a² - x²)/dx
1/[2√(a² - x²)] * (0 - 2x)
= - x/√(a² - x²)
d²y/dx² = - [√(a² - x²) * dx/dx - x * dy/dx]/[√(a² - x²)]²
= - {√(a² - x²) - x * [- x/√(a² - x²)]}/(a² - x²)
= [- (a² - x²) - x²]/[(a² - x²) * √(a² - x²)]
= - a²/(a² - x²)^(3/2)