Цитата(tig81 @ 5.2.2009, 0:46) *

=-(cos^3 t)^(-2)*((cost)^3)'=(-1/cos^6t)*3(cost)^(3-1)*(cost)'=...

(-3(cos t)^2/cos^6 t)*(-sin t)(t)'=3sin t/cos^4 t