b. Find all solutions to the equation m2 − n2 = 56 for
which both m and n are positive integers.
If m2 − n2 = 56, then 56 = (m + n)(m − n). Now
56 = 23 · 7, and by the unique factorization theorem, this
factorization is unique. Hence the only representations
of 56 as a product of two positive integers are 56 =
7·8 = 14·4 = 28·2 = 56· 1. By part (a), m and n must
both be odd or both be even. Thus the only solutions
are either m + n = 14 and m − n = 4 or m + n = 28
and m − n = 2. This gives either m = 9 and n = 5 or
m = 15 and n = 13 as the only solutions.