Derivatives of sin x, cos x, tan x, e^x and ln x | Derivatives of common functions | Khan Academy
Unfortunately there's no proof currently on Khan of the derivatives of sine, cosine, or tangent. Also, the derivative of tangent is secant squared. `1/cos x = sec x` `d/dx (tan x) = 1/cos^2 x = sec^2 x` As for proofs, here's a good proof of the derivative...