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Arithmetic Progressions

Question
CBSEENMA10007561

For what value of n, are the nth terms of two APs: 63, 65, 67, . . . and 3, 10, 17, . . . equal?

Solution

Given A .P.s are :
63, 65, 67, ... and 3, 10, 17, ...
Here, a1 = 63, a2 = 65, a3 = 67, ....
and 6, = 3, b2 = 10, b3 = 17.....
Let d1 and d2 be the common differences of two given A .P.s respectively. Then
d1 = 65 – 63 = 67 – 65 = 2
and d2 = 10 – 3 = 17 – 10 = 7
Now, nth term of the 1st A .P. = a1 + (n – 1)d1
and nth term of the Ilnd A.P. = b1 + (n – 1 )d2
∵ nth terms of the given two A .P.s are equal.
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rightwards double arrow 63 + (n - 1) x 2 = 3 + (n - 1) x 7
rightwards double arrow 63 + 2n -2 = 3 + 7n - 7
rightwards double arrow 2n + 61 = 7n - 4
rightwards double arrow 7n - 2n = 61 + 4
rightwards double arrow   5n = 65
rightwards double arrow   straight n space equals space 65 over 5 equals 13
Hence, the required value of n = 13.