CBSE 10th Class Mathematics
Past year question paper
Year 2020
SET 1, Code Number – 30/5/1
Question Number – 31
Arithmetic progression problem
use the formula of n∧th term
Deepak BoraNewbie
For an A.P., it is given that the first term (a) = 5, common difference (d) = 3 and the n∧th term (an) = 50. Find n and sum of first n terms (Sn) of the A.P
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Given, for an AP
a = 5, d = 3, an = 50
n = ? Sn = ?
an = a+(n – 1)d = 50
5 + (n – 1)3 = 50
(n – 1) 3 = 45
n – 1 = 15
n = 16
Sn = n/2 [a + an]
S16 = 16/2 [a + a16]
S16 = 8 [5 + 50]
= 8 × 55
S16 = 440
n = 16