Further maths help

I was doing a paper and i came across this question:

Angle theta is obtuse and sin theta = root 11/36
Work out the value of cos theta

I managed to get cos theta = 5/6 but its actually -5/6. Could someone explain to me why there's a minus?

I was doing a paper and i came across this question:

Angle theta is obtuse and sin theta = root 11/36
Work out the value of cos theta

I managed to get cos theta = 5/6 but its actually -5/6. Could someone explain to me why there's a minus?

$\cos{(\theta)}$ is negative for obtuse angles, as you will see by considering the cosine graph between $x = 90^{\circ}$ and $x = 180^{\circ}$.

Theta is obtuse, i.e. between 90 and 180.

If you recall your CAST diagram, then sin will be positive, but cos and tan will be negative.