find ds/dt for s = cos^4t (1+t^2)^0.5

(^4 means to the power 4, so cos to power 4, and there's also root (1 plus t squared)

Problem is that I take logs of both sides then try to differentiate using the 1/s ds/dt = rule but get the wrong answer, so a worked answer would be much appreciated. Thanks for any help.