Question

sn=2n^2+n find a15 , a25 , an step by step answer

Answer 1

Answer:

a15=59

and a25=99

Step-by-step explanation:

Sn=2n^2+n

for n=1

S1=2 (1)^2+1

S1=2×1+1

S1=2+1

S1=3=a1

for n=2

S2=2 (2)^2+2

S2=2×4+2

S2=8+2

S2=10=a1+a2

a1+a2 =10

3+a2=10

a2 =10-3

a2=7

a2=a+d

7=3+d

7-3=d

d=4

Therefore,

a15=a+14d

a15=3+14 (4)

a15=3+56

a15=59

Similarly,

a25=a+24d

a25=3+24 (4)

a25=3+96

a25=99

That's all

Hope it helps you