Question

║⊕QUESTION⊕║
Pure mathematics is, in its way, the poetry of logical ideas.
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CLASS 11
SEQUENCES AND SERIES
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The sum of the first 13 terms of an AP is 21 and the sum of the first 21 terms is 13. Show that the sum of the first 34 terms is -34.

Answer 1

Answer:

n = 13, S = 21

Sum = (n/2)( 2a + (n-1)d)

21 = (13/2)( 2a + (13-1)d))

42/13 = (2a + (12)d)

21 = 13a+78d ---------------------(1)

Again,

n = 21, S = 13

13 = (21/2)( 2a + (21-1)d)

26/21 = (2a + (20)d)

13 = 21a+210d -----------------(2)

solving (1) & (2) we get,

a = 283/91 , d = (-68/273)

S(34) = 17(2a + 33d)

putting values of a and d now,

S(34) = (-34)

Answer 2

Answer:

I hope it's help you

SORRY IF IN MY ANSWER ANY MISTAKE