f'(x)=(x^2+1)'(2x^2+8x-5)+(x^2+1)(2x^2+8x-5)'
=2x*(2x^2+8x-5)+(x^2+1)(4x+8)
=4x^3+16x^2-10x+4x^3+8x^2+4x+8
=8x^3+24x^2-6x+8 导数先用乘法法则