Question about the maths of e=mc^2 derivation (using special relativity formula)

Question

Please solve this with every single step, I don't get the addition of the expressions given above, where did the cos phi go? I am just confused as to how these expressions were added, please add them step by step.

Answer 1

Factor out a "1/2L" from the two fractions.

The denominator of the fractions is the same. Add fractions by combining the numerators. A positive (v/c)cosΦ and a negative (v/c)cosΦ combine together to get zero. 1 + 1 = 2.

The 2 in the numerator cancels with the 2 in the denominator.