You will observe that there are two points at which the cross, and the
x's for these points are the values for which sin(x)=cos(x) and so are
the values of a that you require.
can be rearranged to tan(a)=1, so one solution is 45 degrees (pi/4 radian)
The other solution is in the third quadrant (where tan is positive) and so
is 180+45 degrees (pi+pi/4 radian)