# Math Help - obtuse basic trig -ve solution

1. ## obtuse basic trig -ve solution

a is obtuse such that sina=1/5 find the exact value of cosa

I did c^2+s^2=1

and cosa = =+/-(24^0.5)/5

but the aswer is just the negative root,,,is this always the case when obtuse simply or is this answer wrong?

2. Originally Posted by i_zz_y_ill
a is obtuse such that sina=1/5 find the exact value of cosa I did c^2+s^2=1 and cosa = =+/-(24^0.5)/5
but the aswer is just the negative root,,,is this always the case when obtuse simply or is this answer wrong?
Cosine is negative in II. We are given that a is obtuse.