Question

if the sum of n term of an AP is 2n^2 + 5n . then find the sum of first 20th term​

Answer 1

Answer:

Given S

n

=2n

2

+5n

S

n−1

=2(n−1)

2

+5(n−1)

=2(n

2

−2n+1)+5n−5

=2n

2

−4n+2+5n−5

=2n

2

+n−3

Now, t

n

=S

n

−S

n−1

=2n

2

+5n−(2n

2

+n−3)

=2n

2

+5n−2n

2

−n+3

=4n+3

Answer 2

$\huge \underline{ \underline{ \orange{ \bf Solution : }}}$

$\sf given \: \: that \: \: : \: \: sn = 2( {n)}^{2} + 5n$

$\sf s20 = 2( {20)}^{2} + 5(20) = 2(400) + 100$

$\sf = 800 + 100 = 900$

$\therefore \bf s20 \: \: \sf = 900$

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