y=√(a²-x²)+x,本题最好先计算出(√(a²-x²))'=-x/√(a²-x²),这个可以做为结论记住,挺常用的.y'=-x/√(a²-x²) + 1y''=-[√(a²-x²) + x²/√(a²-x²)]/(a²-x²)=-[(a²-x²) + x²]/(a²-x²)^(3/2)=-a²/(a²-x²)^(3/2)
