sagarsabnis wrote:

could anyone please explain in detail??

Take the diagonals of adjacent sides of a cube, so that they make an angle.

Join the ends of these diagonals to get the triangle.

You'll see that the line segment joining the ends of these diagonals will also be the diagonal of a side of a cube.

Hence we have the triangle all 3 sides of which are diagonals of a side of a cube. All diagonals are equal, which means that we have equilateral triangle. Any angle in equilateral triangle is 60 degrees.

Hope it's clear.