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If the sum of first 7 terms of an AP is 119 and that of sum of first 17 terms is 714. Find the sum of first n terms

Posted by Kirandeep Kaur 5 years, 1 month ago

CBSE > Class 10 > Mathematics

1 answers

Yogita Ingle 5 years, 1 month ago

Let first term be a and common diffrence be d

now,

S7 = 7/2(2a+(7-1)d)

119 = 7/2(2a+6d)

119 = 7/2*2(a+3d)

119/7 = a+3d

17=a+3d .........................(1)

Also,

S17 = 17/2(2a+(17-1)d

714 = 17/2*2(a+8d)

714/17 = a+8d

42 = a+8d..........................(2)

Subtracting (1) from (2)

42-17 = a+3d-a-8d

25 = -5d

d = -5

putting d= -5 in (1)

17 = a+ 3(-5)

17 = a-15

17+15 = a

a = 32

Now, a= 32 , d= -5

Sn = n/2{2×32+ (n-1)-5}

Sn = n/2(64-5n+5)

Sn = n/2(69-5n)

Sn = 69n/2 - 5n^2/2

1Thank You

ANSWER

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CBSE > Class 10 > Mathematics