Value of cos a?

[123]

If \(\displaystyle sin a=-\frac{3}{5}\), Which Is The Value Of cos a?

How I do: \(\displaystyle sin^2a+cos^2=1\);
\(\displaystyle cos^2a=1-sin^2a\);
\(\displaystyle cos^2a=1-(-\frac{3}{5})^2\);
\(\displaystyle cos^2a=\frac{16}{25}\);
\(\displaystyle cos a=\sqrt{\frac{16}{25}}=\frac{4}{5}\);

But answer in my book is \(\displaystyle -\frac{4}{5}\) :?:
what's wrong?

 

[Deleted member 4993]

If \(\displaystyle sin a=-\frac{3}{5}\), Which Is The Value Of cos a?

How I do: \(\displaystyle sin^2a+cos^2=1\);
\(\displaystyle cos^2a=1-sin^2a\);
\(\displaystyle cos^2a=1-(-\frac{3}{5})^2\);
\(\displaystyle cos^2a=\frac{16}{25}\);
\(\displaystyle cos a=\sqrt{\frac{16}{25}}=\frac{4}{5}\);

But answer in my book is \(\displaystyle -\frac{4}{5}\) :?:
what's wrong?

Unless someother information is given - the answer should be:

\(\displaystyle cos(a) = \ \pm \ \frac{4}{5}\)