If $p^{th}$ term of an A.P. is $q$ and $q^{th}$ term is $p$, then prove that its $n^{th}$ term is $(p + q - n)$.

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Step 1: $a_p = a + (p - 1)d = q$ ... (i)
Step 2: $a_q = a + (q - 1)d = p$ ... (ii)
Step 3: Solving (i) and (ii): $d = -1$, $a = q + p - 1$
Step 4: $a_n = (q + p - 1) + (n - 1)(-1) = q + p - n$

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