y=ln(x+√x^2+a^2)那么y对x 求导得到y'=1/(x+√x^2+a^2) *(x+√x^2+a^2)'而显然(x+√x^2+a^2)'=1 +2x/2√x^2+a^2=(x+√x^2+a^2)/√x^2+a^2所以就解得y'=1/(x+√x^2+a^2) *(x+√x^2+a^2)/√x^2+a^2=1/√x^2+a^2
