y'=(a²/2)(1/√(1-(x/a)²))(1/a)+(1/2)(√(a²-x²)-x(-2x/2√(a²-x²)))/(a²-x²)=a²/2√(a²-x²)+a²/2(a²-x²)√(a²-x²)
太乱了，加减符号不清。
