Мозг не думает. Какая производная?

f'(x)=-[(x^2+25)/x]'= [(x^2+25)' * x-(x^2+25)] / x^2=[(x^2)' * x-(x^2+25)] /x^2= [2*x*x-(x^2+25)]/x^2=[2x^2-(x^2+25)] / x^2=(2x^2-x^2-25)/x^2=(x^2-25)/x^2