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The 14th term of an A.P.is …

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The 14th term of an A.P.is twice its 8th term. If its 6th term is -8 then find the sum of its first 30 terms

Posted by Khushali . 6 years, 10 months ago

CBSE > Class 10 > Mathematics

2 answers

Nishant Vishwakarma 6 years, 10 months ago

a14=2(a8) a+13d=2(a+7d) a+13d=2a+14d 13d-14d=2a-a -d=a. ---------(1) a6=a+5d=-8-----------(2) putting the vakue of eq (1) in eq (2) -d+5d=-8 4d=-8 d=-8/4 d=-2 Putting the value of d in eq (1) a+5d=-8 a+5(-2)=-8 a-10=-8 a=-8+10 a=2 => s30=30/2(2a+29d) s30=15(2×2+29×-2) s30=15(4-58) s30=15(-48) s30=-720 Ans.

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Naveen Bharti 6 years, 10 months ago

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CBSE > Class 10 > Mathematics