The answer depends on if your problem is (tan x)^3 or tan(x^3)
You can u-sub for either:
(tanx)^3 u=tanx du/dx=sec^2 x
u^3= y
dy/du=3u^2
dy/dx=3(tan x)^2 x sec^2 x
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y=tan (x^3) u=x^3 du/dx= 3x^2
dy/du = sec^2 (u)
dy/dx = sec^2(x^3) x 3x^2
I hope this makes sense to you. Good luck!
