Gujarati
8. Sequences and Series
normal

माना कि $AP ( a ; d )$ एक अनंत समान्तर श्रेणी (infinite arithmetic progression) के पदों का समुच्चय (set) है जिसका प्रथम पद $a$ तथा सर्वान्तर (common difference) $d >0$ है। यदि $AP (1 ; 3) \cap \operatorname{AP}(2 ; 5) \cap AP (3 ; 7)=$ $AP ( a ; d )$ है, तब $a + d$ बराबर . . . . .

A

$150$

B

$154$

C

$155$

D

$157$

(IIT-2019)

Solution

We equate the general terms of three respective

$\text { A.P.'s as } 1+3 a =2+5 b =3+7 c$

$\Rightarrow 3 \text { divides } 1+2 b \text { and } 5 \text { divides } 1+2 c$

$\Rightarrow 1+2 c =5,15,25 \text { etc. }$

So, first such terms are possible when $1+2 c=15$ i.e. $c =7$

$\text { Hence, first term }=a=52$

$d=1 cm (3,5,7)=105$

$\Rightarrow a+d=157$

Standard 11
Mathematics

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