Question

find the sum of the first 30 terms af an AP which is xth term is 2-3n.

Answer 1

Term x = Tx = 2-3n
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For ;
x=1 , T1 = 2-3(1) = -1

x=2 , T2 = 2-3(2) = -4
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a = T1 = -1
d = T2-T1 = -2-(-1) = -1
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Sn = (n/2) [2a+(n-1)d]

S20 = (20/2) [2(-1) + (20-1)(-1)]
= 10 (-2-19)
= 10 × (-21)
= -210
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