# If the Sum of P Terms of an A.P. is Q and the Sum of Q Terms is P, Then the Sum of P + Q Terms Will Be - Mathematics

MCQ

If the sum of p terms of an A.P. is q and the sum of q terms is p, then the sum of p + q terms will be

• 0

•  p − q

• p + q

•  − (p + q)

#### Solution

− (p + q)

$S_p = q$

$\Rightarrow \frac{p}{2}\left\{ 2a + \left( p - 1 \right)d \right\} = q$

$\Rightarrow 2ap + \left( p - 1 \right)pd = 2q . . . . . \left( 1 \right)$

$S_q = p$

$\Rightarrow \frac{q}{2}\left\{ 2a + \left( q - 1 \right)d \right\} = p$

$\Rightarrow 2aq + \left( q - 1 \right)qd = 2p . . . . . \left( 2 \right)$

$\text { Multiplying equation } \left( 1 \right) \text { by q and equation } \left( 2 \right) \text { by p and then solving, we get }:$

$d = \frac{- 2\left( p + q \right)}{pq}$

$\text { Now }, S_{p + q} = \frac{\left( p + q \right)}{2}\left[ 2a + \left( p + q - 1 \right)d \right]$

$= \frac{p}{2}\left[ 2a + \left( p - 1 \right)d + qd \right] + \frac{q}{2}\left[ 2a + \left( q - 1 \right)d + pd \right]$

$= S_p + \frac{pqd}{2} + S_q + \frac{pqd}{2}$

$= p + q + pqd$

$= p + q - \frac{2\left( p + q \right)pq}{pq}$

$= - (p + q)$

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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 19 Arithmetic Progression
Q 2 | Page 51