Question

(cos x)y +(sin y)x=k

Answer 1

If X is a real number then the cosine of x ranges from -1 to +1.

-1<=cos(x)<=+1

Similar thing applies for Sin(x)

-1<=sin(x)<=+1

Also whenever sin(x) is 0, cos(x) is 1 and vice versa.

Cos(x)+Cos(y)=2

=> 1+1=2

Sin(x)+Sin(y)=0.