求导数f(x)=8/(x^2+4)

f'(x)=8(x^2+4)^(-1)
=-8(x^2+4)^(-2)(2x)
=-16x/(x^2+4)^2

f"(x)=-16*[(x^2+4)^2-2(x^2+4)(2x)]/(x^2+4)^4
=-16(x^2+4-4x)/(x^2+4)^3
=-16(x-2)^2/(x^2+4)^3

f(x)=8*(x^2+4)^(-1)
所以f'(x)=8*(-1)*(x^2+4)^(-2)*(x^2+4)'
=-8(x^2+4)^(-2)*2x
=-16x/(x^2+4)^2

f"(x)=-16*{x'*(x^2+4)^2-x*[(x^2+4)^2]'}/(x^2+4)^4
=-16*[(x^2+4)^2-x*2(x^2+4)*(x^2+4)']/(x^2+4)^4
=-16*[(x^2+4)^2-4x^2*(x^2+4)]/(x^2+4)^4
=-16*(x^2+4)[(x^2+4)-4x^2]/(x^2+4)^4
=16(3x^2-4)/(x^2+4)^3

f '(x)=-8（2x)/(x^2+4)^2
带公式