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CBSE Class 10 Answered

An AP is -9 -6 -3 -... How many terms of AP are to be added to get the sum 66
Asked by deviradha979 | 08 Aug, 2019, 08:14: PM
answered-by-expert Expert Answer
-9 -6 -3 -...
Sn = 66, a = -9, d = -6 + 9 = 3
n/2 [2a + (n - 1)d] = 66
n[2 × (-9) + (n - 1) × 3] = 132
n(-18 + 3n - 3) = 132
n(3n - 21) = 132
n(n - 7) = 44
n(n - 7) = 11 × 4
n(n - 7) = 11 × (11 - 7)
Comparing both sides we get n = 11
Total number of terms are 11.
Given terms are 3.
Hence, 8 terms of AP are to be added to get the sum 66
Answered by Sneha shidid | 09 Aug, 2019, 09:50: AM
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