Question

which term of the AP 21,42,63,84,...........is 420

Answer 1

a=21
d=21
$a_{n}$=420
n=?
$a_{n}$=a+(n-1)d
420=21+(n-1)21
(n-1)*21=399
n-1=399/21
n-1=19
n=19+1=20
ans..therefore the 20th term of the AP is 420

Answer 2

an = 420
a = 21 
d = 21
an = a+(n-1)d
420= 21+(n-1)21
399 = (n-1)21
399/21 = (n-10
19 = n-1
19+1 = n
20 = n
20th term is 420
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