The edge lengths of tetrahedral a-bcd are a, e and F are the midpoint of AD and BC respectively. The cosine value of the angle formed by the straight line AF of the different plane to CE is calculated
Let G be the midpoint of be and FG ‖ CE. ∠ AFE be the angle
AF = (√ 3 / 2) a.gf = (√ 3 / 4) a ABG.AG² ; = (7 / 16) a & sup2; (cosine theorem)
Look AFG.cos ∠ AFE = 2 / 3 (cosine theorem)