posted by Ntombie .
Sketch the graph of f(x)=sin(x+30 degrees) and g(x)=cos2x in the same system of axis [-180 degrees;180 degrees]
The graph of cos2x looks just like the graph of cos x, but oscillates twice as fast. So, since cos x crosses at -90 and 90, cos 2x crosses at -135, -45, 45, 135 deg.
The sin graph looks just like sin x, but is shifted to the left 30 deg. (That is, x+30 is zero when x=-30. In effect, just draw a new y-axis at x=-30 deg.)