y' =[1/(x+1)]' =[(x+1)⁻¹]' =(-1)×(x+1)⁻¹⁻¹×(x+1)' =(-1)×(x+1)⁻² ×1=-1/(x+1)² 重要求导法则：(xⁿ)'=nxⁿ⁻¹