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If 3+5+7+........ to n terms/ 5+8+11+.........to …

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If 3+5+7+........ to n terms/ 5+8+11+.........to 10 terms = 7. Find 'n' ?

  • 1 answers

Bindu Agarwal 7 years, 2 months ago

Both the numerator and denominator are the sums of two series up to 'n' terms:-

Since the sum = (n/2)(2a+(n-1)d)

For series in the numerator :-

a=3, d=2, n=n

Therefore numerator = (n/2)*(6+(n-1)2) = (n/2)*(2n+4)

And for the denominator series:-

a=5, d=3, n=10

Therefore denominator = (10/2)*(10+(10-1)3) = (n/2)*(3n+7) = 185

Therefore Acc. to question:-

(n/2)*(2n+4) / (185)   = 7

 = n2 +2n- 1295 = 0

 = n2 + 37n - 35n - 1295 = 0

=n=35 ( Since ncan not be equal to 37)

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