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Question

The sum of first n terms of an A.P. whose first term is 8 and the common difference is 20 is equal to the sum of first 2n terms of another A.P whose first term is -30 and common difference is 8. Find the value of n. 


A
8
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B
9
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C
10
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D
11
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Solution

The correct option is D 11

Given 
a1 = 8                          a2 = -30 
d1 = 20                        d2 = 8

Sn=S2n

n2(2a1+(n1)d1)=2n2(2a2+(2n1)d2)

16+(n-1)20 = 2 [ -60 + (2n-1) 8]

16 + 20n -20 = -120 + 32n -16

12n = 132

n = 11


Mathematics

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