Question

How many terms of the AP 3 7 11 15 gives the sum of 136

Answer 1

$\underline{\underline{\mathfrak{\Large{Solution:}}}}$

We know,
Sn=n/2{2a+(n-1)d}

By the given information,
a=3
d=7-3=4
Sn=136
n=?

Substituting these is this equation.

136=n/2{2×3+(n-1)4}

272=6n+4n^2-4n

272=2n+4n^2

4n^2+2n-272=0

By solving we get
n=(-2+or-√1092)/8

$Hope this helps#$

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