.
sin^-1 (3÷5) + sin^-1 (5÷13) = sin^-1 (56÷65)
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Thus we need to prove that arcsin + arcsin = arcsin.
Proof
Let = arcsin and = arcsin.
Thus = and is in Quadrant 1; = and is in Quadrant 1 also.
We need to prove that = and + is in Quadrant 1.
Since = , you have = = (make all intermediate calculations yourself)
Since = , you have = = (make all intermediate calculations yourself)
Now apply the formula for :
= = = = .
Thus the first half of the statement is proved.
Next calculate :
= = = = .
Since > 0, is in the quadrant 1.
The proof is completed.