Q. 393.9( 22 Votes )

# If the sum of first m terms of an A.P. is the same as the sum of its first n terms, show that the sum of its first (m + n) terms is zero.

Answer :

Let the first term be a and common difference of the given AP is d.

Given: Sm = Sn

2am + md(m – 1) = 2an + nd(n – 1)

2am – 2an + m2d – md – n2d + nd = 0

2a (m – n) + d[(m2 – n2) – (m – n)] = 0

2a (m – n) + d[(m– n)(m + n) – (m – n)] = 0

(m – n) [2a + {(m + n) – 1}d] = 0

2a + (m + n – 1)d = 0 [ m – n ≠ 0]…(i)

Now,

[using (i)]

Sm+n = 0

Hence Proved

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