Question

Find the sum of first 25 terms of following progression whose n'h term is given

(i) a = 7 -3n​

Answer 1

Step-by-step explanation:

an = 7 - 3n

n = 25

a = 7 - 3(1)

a = 7 - 3

a = 4

an = n + ( n - 1) x d

a 25 = 25 + (25 - 1) d

to find d we should find a

t1 = 4

t2 = 7 - 3(2)

t2 = 7 - 6

t2 = 1

d = t2 - t1

= 4 - 1

= 3

a25 = 25 + (25-1) x 3

a25 = 25 + 24 x 3

a25 = 97

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