Question 950974
You know that,
{{{cos^2(theta)+sin^2(theta)=1}}}
{{{cos^2(theta)+(7/17)^2=1}}}
{{{cos^2(theta)=(17/17)^2-(7/17)^2}}}
{{{cos^2(theta)=(289-49)/(17)^2}}}
Since we know it's in the first quadrant, we only need the positive square root,
{{{cos(theta)=sqrt(240)/17}}}
{{{cos(theta)=(4/17)sqrt(15)}}}