Here's the question you clicked on:
adnanchowdhury
Trigonometric Identities question: http://d.pr/i/FsnP I got my answer to be 3/2 but the mark scheme says -3/2.
\[3\cos2 \beta+19\cos \beta+13=0\] \[Use ~~~~\cos2\beta=2\cos^2\beta-1\] \[3(2\cos^2\beta-1)+19\cos \beta+13=0\] \[6\cos^2\beta-3+19\cos \beta+13=0\] \[6\cos^2\beta+19\cos \beta+10=0\] \[(2\cos \beta+5)(3\cos \beta+2)=0\] \[\cos \beta=-5/2, \cos \beta =-2/3\]
Thanks. I made a silly mistake.