We recognise `y=x^2/10` as a parabola, that we met before.

We draw the two curves `y=x^2/10` (green, dotted) and `y = -sin\ πx` (blue) on the same set of axes.

composite trigo graph

The sum of the y-ordinates of the two curves is shown next:

composite trigo graph

Expanding the domain (that is, zooming out) gives us a better view of the parabola added to the sine curve:

composite trigo graph examle - zoomed out

Easy to understand math videos:
MathTutorDVD.com