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an=4 ,d=2, sn=-14 find 'n' and …

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an=4 ,d=2, sn=-14 find 'n' and 'a' .

Posted by Rajput Singh 6 years, 10 months ago

CBSE > Class 10 > Mathematics

1 answers

Khushboo Giri 6 years, 10 months ago

an=4 a+(n-1)d=4 a+2n-2=4 a+2n=6 a=6-2n-------(1) Sn=-14 n/2(2a+(n-1)d)=-14 n/2(2a+(n-1)2)=-14 n(a+n-1)=-14 n(6-2n+n-1)=-14-----from (1) n(5-n)=-14 5n-n²=-14 n²-5n-14=0 n²-(7-2)n-14=0 n²-7n+2n-14=0 n(n-7)+2(n-7)=0 (n+2)(n-7)=0 Therefore, n=7 Putting the value of n in equation (1) a=6-2×7 a=6-14 a=-8

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