The common difference of an A.P.is -2. Find its sum,
if its first term is 100 and the last term is - 10.
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Step-by-step explanation:
d =-2
a= 100, l = - 10
l = a + (n-1) d
l = 100 + (n-1)d
-10 = 100 + (n-1)(-2)
2 (n-1) = 110
n-1 = 55
n= 56
S = n/2 [2a + (n- 1)d]
= 56/2 [2×100 + (56-1)(-2)]
= 28 [200-110]
= 28× 90
= 2520
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