Which term of an A.P. 16, 11, 6, 1,... is -54 ?
Solution :
Let -54 be the nth term of the sequence
tn = -54
a + (n - 1)d = -54
a = 16 , d = 11 - 16 ==> -5
16 + (n - 1)(-5) = -54
(n - 1)(-5) = -54 - 16
(n - 1)(-5) = -70
n - 1 = -70/(-5) ==> 14
n = 14 + 1
n = 15
Hence 15th term of the given sequence is -54.