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प्रश्न
Which term of the A.P. 21, 42, 63, 84, ... is 420?
उत्तर
In the given problem, we are given an A.P and the value of one of its term.
We need to find which term it is (n)
So here we will find the value of n using the formula, `a_n = a + (n -1)d`
Here, A.P is 21, 42, 63, 84
`a_n = 420`
a = 21
Now
Common difference (d) = `a_1 - a`
= 42 - 21
= 21
Thus, using the above mentioned formula
`a_n = a + (n -1)d`
`420 = 21 + (n - 1)21`
420 - 21 = 21n - 21
399 + 21 = 21n
`n = 420/21
n = 20
Thus n = 20
Therefore 420 is the 20th term of the given A.P
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