8)
[tex]\it x,y\in(0,\ \dfrac{\pi}{2})\\ \\ sin x=\dfrac{3}{5} \Rightarrow cosx=\sqrt{1-sin^2x}=\sqrt{1-\Big(\dfrac{3}{5}\Big)^2}=\sqrt{1-\dfrac{9}{25}}=\sqrt{\dfrac{16}{25}}=\dfrac{4}{5}[/tex]
Analog se determină cosy, apoi se folosește formula:
[tex]\it cos(x+y)=cosxcosy - sinxsiny[/tex]
