Question 938450
Sine is positive, tangent is negative, so cosine must be negative. 
{{{(1/2)^2+cos(theta)^2=1}}}
{{{cos(theta)^2=1-1/4}}}
{{{cos(theta)^2=3/4}}}
{{{cos(theta)=0 +- sqrt(3)/2}}}
From above,
{{{cos(theta)=-sqrt(3)/2}}}