\left \{ {{20sin(x)+12cos(y)=8} \atop {20sin(x)+25cos(y)=15}} \right.
вычитаем:
13cos(y)=7\\cos(y)=\frac{7}{13} \\y=(+-)arccos(\frac{7}{13})+2\pi k
k∈Z
-------\\\left \{ {{25sin(x)+15cos(y)=10} \atop {12sin(x)+15cos(y)=9}} \right.
вычитаем:
13sin(x)=1\\sin(x)=\frac{1}{13}\\x=(-1)^karcsin(\frac{1}{13} )+\pi n
n∈Z