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for the first triangle , sin A = b/r and cos A = a/r

as for the second triangle , why is sin (90+A) = a/r = cos A

and why is cos(90+A) = -b/r = -sin A

Im confused ... how can you compute the trig ratio for an angle greater than 90 degress ? Why not take the ratio for the acute angle (180 - (90+a))

Pls Help .