[(2+5x)^10]'=10(2+5x)^9*(2+5x)'=50(2+5x)^9
所以f'(x)=[x*50(2+5x)^9-1*(2+5x)^10]/x^2
=[50x(2+5x)^9-(2+5x)^10]/x^2
=(45x-2)(2+5x)^9/x^2