if theta is very small why do we use?

A. |\

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.B.|_____\ C

I'll use a right triangle to show the principles involved. If the angle at A were Theta, then Sin theta would equal BC/AC. Tan theta would equal BC/AB. We also know that

(AB)^2 + (BC)^2 = (AC)^2...........Pythagorean theorem

Now, as theta gets smaller (approaches 0), BC gets smaller (approaches 0)

As BC approaches 0, AB approaches AC (look at Pythagorean theorem/replace BC with 0)

So, Sin approaches Tan

Hope you could follow that.

Not Necessary to be small . we use it to found out the specific angle in geometry and sometimes in higher math we use it for to find matrix ....