ABCDEFGH, refer to image in the link, is a cube. Find the sine of ∠GAC.
GAC will form a right triangle
Let GC = the side of the cube = S
Then AC will = S√2
And GA = S√3
So.....GA is the hypotenuse and AC and GC will form the legs
sin GAC = GC / GA = S / [ S√3 ] = 1 /√ 3
[ To find the angular measure use arcsin (1/ √3) ≈ 35.26° ]