We have
am = a + (m – 1)d = n, …… (1)
and an = a + (n – 1)d = m …….. (2)
Solving (1) and (2), we get
(m, n) d = n – m, or d = -1,
and a = n + m – 1 ……. (3)
Therefore ap = a + (p – 1)d ……. (4)
= n + m – 1 + (p – 1) (-1)
= n + m – p
Hence, the pth term is n + m – p.