1. The problem statement, all variables and given/known data y= A(x-sin(x)) with A as a constant. 2. Relevant equations dy/dx = A(1-cos(x)) ?? 3. The attempt at a solution If I am thinking about this correctly, one can just differentiate the function as I have, and argue that when the gradient (dy/dx) is less than zero, the function is decreasing. So from this: A(1-cos(x))<0 so 1-cos(x)<0 cos(x)>1 which never happens for any x, including negative x since cos is an even function. Is this the right way of doing this? I'm very rusty on proofs etc. Thanks in advance!