I don't understand how to do this type of question? If 9cos^2 theta +3cos theta=0, where 90 degrees <theta<180 degrees, what is the value of theta, correct to the nearest tenth of a degree. Could someone please explain. Thanks
9cos^2(t)+3cos(t)=0
subtract 3cos(t) from both sides
9cos^2(t)=-3cos(t)
divide both sides by 3cos(t)
3cos(t)=-1
cos(t)=-1/3
t=arccos(-1/3)