If mth term of an A.P. is n and nth term in m, then its r th term is
m + n – r
m + n + r
m – n + r
r – (m + n)
am=n,an=m⇒(m−n)d=an−an=−(m−n)⇒d=−1
Now, ar−am=(r−m)(−1)
⇒ar=am−r+m=m+n−r