Question

How many terms of a.p 2'5'8........should be added to give sum 155

Answer 1

a= 2
d= 3

Sn= 155
n[2a+(n-1)d] / 2 = 155
=> n[4 +(n-1)3] = 310
=> n(4+3n-3) = 310
=> n(1+3n) = 310
=> n + 3n^2 = 310
=> 3n^2 + n - 310 = 0
=> 3n^2 - 30n + 31n - 310 = 0
=> 3n(n-10) + 31(n-10) = 0
=> (n-10)(3n+31) = 0

n= 10 and - 31/3

No. of terms cannot be negative

n= 10

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