Question 792053
In an arithmetic sequence the 8th term is twice the 4th term and the 20th term is 40. Find the common difference and the sum of the terms from 8th to the 20th inclusive..

a= first term
n= number of terms
d = common difference

8th. Term
T8 = a+7d

T4 = a+3d

T8=2*T4

a+7d=2(a+3d)
a+7d=2a+6d
a-d=0
a=d

T20 = a+19d

-
Therefore T4 = 4a
T8=8a
T20=20a
40=20a
a=2
but a=d
so d=2

S20= 10(4+19*2)

=420

S10= =5(4+18)
S10 = 110

S20-S10= 420-110= 310