Question

If the nth term of an ap is 7-3n,find the sum of 25 terms

Answer 1

Tn = 7 - 3n

=> a + (n-1)d = 4 + 3 - 3n

=> a + (n - 1)d = 4 - 3( n - 1)

On Comparing both sides, we get

a = 4

d = - 3

Now,

Sn = 25/2 [ 2a +(25 - 1) d]

= 25/2 [ 2×4 + (24) (-3)]

= 25/2 [ 8 - 72]

= 25/2 [ -64]

= 25 × (-32)

= - 800

Answer 2

Answer:

Step-by-step explanation:

Given :-

n = 25

a(n) = 7 - 3n

To Find :-

Sum of 25 terms.

Formula to be used :-

S(n) = n/2[2a + (n - 1)d]

Solution :-

Taking n = 1, 2, 3, ........, we get

a(1) = 7 - 3 × 1 = 4

a(2) = 7 - 3 × 2 = 1

a(3) = 7 - 3 × 3 = - 2

A.P is 4, 1, - 3, ........

Here, a = 4, d = 1 - 3 = - 3

Since, S(n) = n/2[2a + (n - 1)d]

⇒ s(25) = 25/2[2 × 4 + (25 - 1) (- 3)]

⇒ s(25) = 25/2[8 + 24 (- 3)]

⇒ s(25) = 25/2(8 - 47)

⇒ s(25) = - 800

Hence, the sum of 25 terms is - 800.