[Math] n easy way to quickly prove (or memorize) inverse trig formulas such as $ \arcsin(a) = \arctan(\frac{a}{\sqrt{1-a^2}}) $

trigonometry

Is there an easy way to quickly prove these formulas?

If not, is there any easy mnemonic way to memorize them fast?

$$\begin{align}
\arcsin(a) &= \arctan\left(\frac{a}{\sqrt{1-a^2}}\right) \\[4pt]
\arccos(a) &= \operatorname{arccot}\left(\frac{a}{\sqrt{1-a^2}}\right) \\[4pt]
\arctan(a) = \arcsin\left(\frac{a}{\sqrt{1+a^2}}\right) &= \arccos\left(\frac{1}{\sqrt{1+a^2}}\right) = \operatorname{arccot}\left(\frac{1}{a}\right) \\[4pt]
\operatorname{arccot}(a) &= \arccos\left(\frac{a}{\sqrt{1+a^2}}\right)
\end{align}$$

P.S. Wikipedia desribes it here

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Best Answer

For the first one draw a right angled triangle as below.enter image description here

Now $\sin x = a \implies x = \sin^{-1}a = \tan^{-1}\frac{a}{\sqrt{1-a^2}}$

Do similarly for the other cases.

enter image description here