Question

Find the sum of first 25 terms of an AP whose nth term is 1 – 4n.​

Answer 1

Step-by-step explanation:

Answer:

−1275

Step-by-step explanation:

nth term of AP = a_n=1-4na

n

=1−4n

Substitute n =1

a_1=1-4(1)=-3a

1

=1−4(1)=−3

Substitute n =2

a_2=1-4(2)=-7a

2

=1−4(2)=−7

Substitute n = 3

a_3=1-4(3)=-11a

3

=1−4(3)=−11

So, first term = a = -3

Common difference = d = -7-(-3)=-11-(-7)=-4

Formula of sum of first n terms = S_n=\frac{n}{2}(2a+(n-1)d)S

n

=

2

n

(2a+(n−1)d)

Substitute n = 25

S_{25}=\frac{25}{2}(2(-3)+(25-1)(-4))S

25

=

2

25

(2(−3)+(25−1)(−4))

S_{25}=−1275S

25

=−1275

Hence the sum of first 25 terms is −1275

Answer 2

Answer:

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Step-by-step explanation:

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