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The 5th & 15th terms of ap are 13 & -17 respectively find the sum of the first 21 terms of the ap

Posted by Kulwinder Matharu 5 years, 11 months ago

CBSE > Class 10 > Mathematics

1 answers

Deepanshu Tyagi 5 years, 11 months ago

Given, A+4d=13 ___(1) and a+14d=-17___(2) Subtracting (1) from (2):- A+14d=-17 A+4d=13 - - - __________ 10d=-30 __________ D=-3 Putting this value in eq.(1):- A+4d=13 A+4*(-3)=13 A-12=13 A=25 Sn=n/2 [2a+(n-1)d] S²¹=21/2 [2*25 +(21-1)-3] S²¹=21/2 [50 - 60] S²¹=21/2 *(-10) S²¹=-105

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CBSE > Class 10 > Mathematics