中四附加數

Let t be the travelling time.

AC=30root3 t
BC=30t

Extend CB to O which is perpendicular to A
Let OB=y, then
AB=2y
AO=root3 y
By pythagoras' theorem of triangle ACO,
...
1800t^2-60yt-4y^2=0
then t=1/15y

AO/CA = root3 y/ 30 root3 t = cos (30+ x) where x is the angle
1/2 = cos (30+x)
x = 30 degrees

so the direction is N30degreeE