Look at a point on a sinusoidal curve (y=sin(x)) with a tangent line.
Then, look at a point on the negative of your sinusoidal curve (y=-sin(x)) that is a mirror image of your first point.
Then, notice that these mirror image points have mirror image tangent lines.
Observe the definite integral of sine, or the area between the function sin(x) and the x axis, and how it changes between different bounds by dragging the boundaries, a and b. What happens to the area when the interval is 2Π? Why?