Question

Please..... help or say answer at tharun_rai insta​

Answer 1

Answer:

n²+6n

Step-by-step explanation:

S4= (4/2)[2a+(4-1)d]

40/2= 2a + 3d

2a+ 3d =20 ---- 1)

S14=(14/2)[2a+(14-1)d]

280/7= 2a +13d

2a + 13d = 40

2a+ 3d= 20

-      -       -

-------------------

10d= 20

d=20/10=2

Now putting d's value in 1

2a +3(2)=20

2a=20-6=14

a=14/2=7

Therefore Sn = (n/2)[2a +(n-1)d]

                     = (n/2)[2(7) + (n-1)2]

                      = n(7+n-1)

                       =n(n+6)

                     =n²+6n