4cos(5x+3y+2z)-5x-3y-2z=0
dz/dx=- (dF/dx)/(dF/dz)=- (-4*sin(5x+3y+2z)*5-5)/(4*(-sin(5x+3y+2z)*2-2)= -(20sin(5x+3y+2z)-5)/(8sin(5x+3y+2z)-2)
dz/dy= - (dF/dy)/(dF/dz)= -(4*(-sin(5x+3y+2z)*3-3)/(-4*sin(5x+3y+2z)*2-2)= -(12*sin(5x+3y+2z)-3)/(8*sin(5x+3y+2z)-2)