If the pth term of an AP is q and its qth term is p then its mth term is:
pth term = a + (p – 1) d = q ... (i)
qth term = a + (q – 1) d = p ... (ii)
Subtracting (ii) from (i)
(p – 1) d – (q – 1) d = q – p
→pd – d – qd + d = q – p
→d (p – q) = q – p
→ d = -1
a = q – (p – 1)d = q + (p – 1)
So, mth term = q + p – 1 – (m – 1) = q + p – 1-m + 1 = p + q – m