Trigonometric Identities and formulas There are so many identities like $\sin2θ$, $\cos2θ$, $\tan2θ$, $\sin(θ/2)$, $\cos(θ/2)$ and $\tan(θ/2)$. there are other formulas too like $\cos(α-β)$, $\sin(α-β)$ etc and yes the sum and product formulas of trigonometric function...
I am stuck in memorizing all. Is there any simple trick to memorize these formulas and identities?

Prove all the formula by urself using concept of geometry especially the formula of compound angles. When to solve problems,first take help by directly seeing the formula. Don't cram them. Before sleeping try to memorize them . [Some techniques :    $sin2\theta$ & $cos2\theta$ when  expressed in terms of $tan\theta$ ,then in denominator, there will be always $+$ sign but in formula of $tan2\theta$, the denominator carries $-$ sign (tan is a hospitable guy!) . When $cos2\theta$ is expressed in terms of $sin^2\theta$ then sine takes $-$ sign but when expressed in terms of $cos^2\theta$ ,then cosine takes $+$ formula( it is cos' formula ; why should it be negative ?) . In formula of $sin3\theta$ it takes $3sin\theta$ as its 1st term followed by $4sin^3\theta$ . On the other hand, $cos3\theta$ takes $4cos^3\theta$ followed by $3cos\theta$ ( Cos is greedy; it takes the term beginning with 4 and then takes the term beginning with 3. But sin is sober ; it takes first the  term beginning with 3  and then the other term).] .
