Question

if cos feta=-3/5 and pi 14 years ago

Answer 1

you need the "half angle" formula for cosine. do you know it?

Answer 2

it is
\[cos(\frac{\theta)}{2}=\pm\sqrt{\frac{1+cos(\theta)}2}\]

Answer 3

so just replace theta by \[\frac{-3}{5}\]

Answer 4

you know by the condition
\[\pi<\theta<\frac{3\pi}{2}\] that you are in quadrant II so cosine is negative there. make sure you take the negative square root, not the positive.

Answer 5

excuse me i did not mean replace theta by -3/5 i meant replacse cos(t) by -3/5

Answer 6

you get
\[\sqrt{\frac{1-\frac{3}{5}}{2}}\]

Answer 7

=\[\sqrt{\frac{\frac{2}{5}}{2}}\]

Answer 8

=\[-\sqrt{\frac{2}{10}}\]

Answer 9

=\[\sqrt{\frac{1}{5}}\] if my arithmetic is right