English
Hindi
8. Sequences and Series
hard

બે સમાંતર શ્રેણીઓનાં $n$ પદોના સરવાળાનો ગુણોત્તર $2n + 3 : 6n + 5$ હોય, તો તેના $13$ મા પદોનો ગુણોત્તર....... છે.

A

$53 : 155$

B

$27 : 77$

C

$29 : 83$

D

$31 : 89$

Solution

$ \frac{{{S_{{n_1}}}}}{{{S_{{n_2}}}}} = \frac{{2n + 3}}{{6n + 5}}$

$\,\therefore \,\,\,\,\frac{{\frac{n}{2}[2{a_1} + (n – 1){d_1}]}}{{\frac{n}{2}[2{a_2} + (n – 1){d_2}]}}= \frac{{2n + 3}}{{6n + 5}}$

$\therefore \,\,\,\frac{{{a_1} + \left( {\frac{{n – 1}}{2}} \right)\,{d_1}}}{{{a_2} + \left( {\frac{{n – 1}}{2}} \right){d_2}}} = \frac{{2n + 3}}{{6n + 5}}$

${\text{n  =  25}}$ લેતાં $\frac{{{a_1} + 12{d_1}}}{{{a_2} + 12{d_2}}} = \frac{{53}}{{155}}$

$\therefore \frac{{{t_{13}}}}{{{{t'}_{13}}}} = \frac{{53}}{{155}}$

 

Standard 11
Mathematics

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