Question

the sum of first 13 terms of an AP is 21 and the sum of first 21term is13 .determine the sum of first 34 terms

Answer 1

a13 = 21
a + 12d = 21 - - - (1)equ
a21 = 13
a + 20d = 13 - - - (2)equ

a+ 20d = 13
a + 12d = 21
___________
8d = -8
d = -8/8
d = ( -1 )
putting d= ( - 1 ) in equ 1
a+ 12 × (-1) = 21
a - 12 = 21
a = 21 - 12
a = (-9)

a34 = a + 33 d
= (-9) + 33 × (-1 )
= (-9) - 33
= (-42)