解y=√x^2是偶函数,注意a＞0
则 ∫( √x^2)dx(a>0)(上限a ,下限-a)
= 2∫( √x^2)dx(a>0)(上限a ,0)
= 2∫/x/dx(a>0)(上限a ,0)
=2∫xdx(a>0)(上限a ,0)
=2*（1/2*x²）(上限a ,0)
=x²(上限a ,0)
=a²-0²
=a²