Question 924526
From the identity,
{{{sin^2(beta)+cos^2(beta)=1}}}
{{{(1/8)^2+cos^2(beta)=1}}}
{{{cos^2(beta)=63/64}}}
{{{cos(beta)=0 +- sqrt(63)/8}}}
However since there is a limit on {{{beta}}}({{{180< beta <270}}}), you choose the negative value because in that range, the cosine is negative.
However, in that range, the sine is also negative.
Are you sure it isn't {{{sin(beta)=-1/8}}}??