the sum of a series in A.p.is 72 the first term is 17 and the common difference -2.the number of terms is
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Answer:
The number of terms is either 6 or 12
Step-by-step explanation:
Sn = 72
a = 17
d = - 2
Sn = n/2 [ 2a + (n-1)d]
72 = n/2 [34 + (n-1) (-2)]
72 = n[17 - (n-1)]
72 = n[ 18 - n]
72 = 18n - n²
n² - 18n +72 = 0
n² -12n -6n +72 = 0
n (n - 12) - 6 (n - 12) = 0
( n - 12 ) ( n - 6 ) = 0
∴ n= 12. or n = 6
∴ The number of terms is either 6 or 12
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