The angle between the vectors that connect ther carbon atom to the hydrogen atoms in the methane molecule can be calculated using the dot product or the cross product rule. In the case of the dot product, we get the correct answer, which is 109.5 degree, but if we use the cross product, the answer might be 70.5 degree. Why?

The angle between the vectors that connect ther carbon atom to the hydrogen atoms in the methane molecule can be calculated using the dot product or the cross product rule. In the case of the dot product, we get the correct answer, which is 109.5 degree, but if we use the cross product, the answer might be 70.5 degree. Why?