1.

How many terms of the AP 21, 18, 15, … must be added to get die sum 0?

Answer»

AP is 21, 18, 15,…

a = 21,

d = 18 – 21 = -3

Sum of terms = Sn = 0

(n/2) [2a + (n – 1)d] = 0

(n/2) [2(21) + (n – 1)(-3)] = 0

(n/2) [45 – 3n] = 0

[45 – 3n] = 0

n = 15 (number of terms)

Thus, 15 terms of the given AP sums to zero.



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