√(1+x²)-1=[√(1+x²)-1][√(1+x²)+1]/[√(1+x²)+1]=x²/[√(1+x²)+1]x→0则2/[√(1+x²)+1]→1x²/[√(1+x²)+1]/(x²/2)→1所以等价无穷小是 x²/2...