Question

2) Findi S20 if first term is 4 and common difference is3
​

Answer 1

Answer:

96 .......☺..........

Answer 2

Answer:

(i) If a

n

=3−4n, show that a

1

,a

2

,a

3

,… form an A.P. Also find S

20

From the question it is given that,

n

th

term is 3+4n

So, a

n

=3−4n

Now, we start giving values, 1,2,3,… in the place of n, we get,

a

1

=3−(4×1)=3−4=−1

a

2

=3−(4×2)=3−8=−5

a

3

=3−(4×3)=3−12=−9

a

4

=3−(4×4)=3−16=−13

So, The numbers are −1,−5,−9,−13,…

Then, first term a=−1,

common difference d=−5−(−1)=−5+1=−4

We know that,

S

20

=(n/2)(2a+(n−1)d)

=(20/2)((2×(−1))+(20−1)×(−4))

=10(−2+(19×(−4)))

=10(−2−76)

=10(−78)

=−780