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

1.	How many terms of an A.P. 9,17,25....... must be taken to give a sum of 636
Answer» a = 9d = 17-9 = 8sum = 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 +4n2or 4n2\xa0+ 5n -\xa0636 = 04n2\xa0+ 53n - 48n -\xa0636 = 0n (4n-53) - 12 (4n-53) = 0(4n-53) (n-12) = 0n= 53/4 as number of terms can\'t be in decimals. so this can\'t be our answertherefore, n = 12 is correct answer\xa0

Discussion