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How many terms of an A.P. 9,17,25....... must be taken to give a sum of 636

Posted by Venugopal Hs 5 years, 8 months ago

CBSE > Class 10 > Mathematics

1 answers

Kamlesh Thakur 5 years, 8 months ago

a = 9

d = 17-9 = 8

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

636 = n/2 (18+ (n-1)8)

636 = n/2 (18+8n-8)

636 = n/2 (10+8n)

636 = n (5+4n)

636 = 5n +4n²

or 4n² + 5n - 636 = 0

4n² + 53n - 48n - 636 = 0

n (4n-53) - 12 (4n-53) = 0

(4n-53) (n-12) = 0

n= 53/4 as number of terms can't be in decimals. so this can't be our answer

therefore, n = 12 is correct answer

0Thank You

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