We have the formula (sin x)^2 + (cos x)^2 = 1;
Then, sin α = [tex] \sqrt{1- ( \frac{4}{5}) ^{2} } = \frac{3}{5} ;[/tex]
cos β = [tex] \sqrt{1- ( \frac{4}{5}) ^{2} } = \frac{3}{5} ;[/tex]
We apply the formula sin ( α + β ) = sin α x cos β + sin β x cos α = (3/5)x(4/5) + (4/5)x(3/5) = 12/25 + 12/25 = 24/25;
