If the 3rd and the 5th term of an arithmetic progression are 13 and 21, what is the 13th term?
A) 53 B) 49 C) 57 D) 61
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A3 = 13
a + 2d = 13
A5 = 21
a + 4d = 21
a + 2d = 13 – ( 1 )
a + 4d = 21 – ( 2 )
__________
– 2d = – 8
d = 8/2
d = 4
put d = 4 in ( 1 )
a + 8 = 13
a = 13 – 8
a = 5
A13 = a + 12d
= 5 + 12 (4)
= 5 + 48
= 53
( A ) is the correct answer.
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