Math question??? Mensuration???

Question

If V is the volume of a cuboid of dimensions a, b, c and S is its surface area, then prove that
1/V = 2/S(1/a + 1/b + 1/c)

Answer 1

The volume V=abc
Surfce area S=2(ab+bc+ca)
Therefore,2/S(1/a+1/b+1/c)
=2/2(ab+bc+ca )](ab+bc+ca)/abc
=1/abc=1/V proved

Answer 2

2/s(1/a + 1/b + 1/c)= 2/2(ab+bc+ac)(1/a+1/b+1/c)
=(ab+bc+ac)(1/a+1/b+1/c)
=b+a+ab/c+bc/a+c+b+c+ac/b+a)
now just find lcm and u should arrive at the answer