So I'm trying to find out if the curve, cos(x - y) = xy, is decreasing or not (whether dy/dx > 0 or dy/dx

I found the derivative of this function, but how would I plug in a number into the derivative since there's both x and y? E.g. pi/4 which is between 0 and pi/2?

Practice of du [ cos (x-y)= xyJ'> [cosGe-y)] =>Apply dudx where f= cos (a ) - sin ( x - y ) . (* * dy da ) u = 2 - y [ x-y ] = [x] -[x]' -> [ xy ] = 1 . y + x . dy/dx 1 - dy/d x - y + x dy x = > - sin ( x - y ) (1 - dy/ dxx) = y+ x dy/dx isolate dy/ex : => Apply a (b - c ) = ab-ac where a = - sin (x-y) b=1 c = do/dx ab = - sin ( x - y ) - 1 = - sin (x-y ) ac = - sin ( x - y ) . dy as = dy ( - sin ( x - y ) ) ab- ac = > - sin (x-y) - (-sin (x-y)) atax = ) - sin ( x-y ) + sin(x-y) dy/dx = y + xdy/dze + sin (x-y ) + sin (x -y ) = > sin ( x -y) dy/d x = y + xx dyx+ sin (x-y) - x dy/ ex - 26 dy/ d x = ) sin (x -y ) dyex ady = y + sin (2-y) = > dy ( sin ( x - 4 ) - x ) = y+ sin (xe -y) dx = ) dy = y+ sin ( x -y ) dx sin ( x - y ) - x At pi/ 4 - y + sin