Question

the fourth term of an ap is 10 and the 11th term of 8 x 8 6 8 x 4 find the sum of first 20 terms of the progression​

Answer 1

Answer:

Step-by-step explanation:

T4=10

T11=3(T4)+1

T11=3(10)+1

T11=31

Now, Tn=a+(n-1)*d

Here a=first term, n=term, d=common differences

T4=a+(4–1)*d

10=a+3*d

a+3*d=10…………….. (1)

Tn=a+(n-1)*d

T11=a+(11–1)*d

31=a+10*d

a+10*d=31…………..(2)

Subtracting (2) from (1)

We get, 7*d=21

d=21/7

d=3

Substituting d=3 in eq (1)

a+3*d=10

a+3*3=10

a+9=10

a=10–9

a=1

We Know that Sn=n/2[2*a+(n-1)*d]

S20=20/2[2*1+(20–1)*3]

S20=10[2+19*3]

S20=10[2+57]

S20=10[59]

S20=590

20th term is 590