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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. Fnd n. 


Solution

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

Sn1=2S2n2

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
RD Sharma
Standard X

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