Question

S4=40,S14=280, find Sn​

Answer 1

Answer:

if the series is AP

then,

s4=4a+6d=14

 or, 2a +3d=7----------------eqn 1

s14=14a+91d=280

or, 2a+13d=40-------------eqn 2

solving eqn1 and eqn 2

d=33/10

a=-29/20

sn=n/2[2a+(n-1)*d]=n/2[(33n/10)-(31/5)]=(33n^2/20)-(31n/10)

Answer 2

Answer:

Step-by-step explanation: