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Sum of 7 term 49 and sum of 17 term is 289 find sn

Posted by Rohit Yadav 6 years, 1 month ago

CBSE > Class 10 > Mathematics

1 answers

Kriti Anand 6 years, 1 month ago

S7=7/2(2a+6d). 49=7/2(2a+6d). 2a+6d=49*2/7. 2(a+3d)=14. a+3d=7. a=7-3d-------------(1). S17=17/2(2a+16d). 289*2/17=2(a+8d). 289/17=a+8d. a+8d=17---------------(2) Putting value of a in eq. (2). 7-3d+8d=17. 7-5d=17. -5d=10. d=-2. a=7-3*-2. a=13. Now, Sn=n/2(2a+(n-1)d). Sn=n/2(2*13+(n-1)-2). Sn=n(14-n).

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