Trigonometric form (school)

[gefest_md]

Find the trigonometric form of the number $\sin\alpha-i\cos\alpha$. The answer is $\cos\left(-\frac{\pi}{2}+\alpha\right)+i\sin\left(-\frac{\pi}{2}+\alpha\right)$. But I can reach this just for $\sin\alpha>0$. Is the answer correct?

[Алексей К.]

Yes.

-- 31 май 2012, 19:26:56 --

gefest_md в сообщении #579007 писал(а):

But I can reach this just for $\sin\alpha>0$

Why???