Question 1072109
That means each of the other two angles are measure 80 degrees.  The shorter diagonal is opposite either of these 80 degree angles.


d for the diagonal
{{{d^2=10^2+5^2-2*5*10*cos(80)}}}


{{{d=sqrt(100+25-100*cos(80))}}}


{{{d=sqrt(125-100*cos(80))}}}


{{{highlight(d=5sqrt(5-4*cos(80)))}}}