Let the common difference of both the
A.P.’s = d
and First term of Ist A.P. = a
also first term of 2nd A .P. = b
Then, 200th term of 1st A .P. = a + 99d
and 100th term of IInd A .P. = b + 99d
Difference between their 100th term = 100
(given)
⇒ (a + 99d) – (b + 99d) = 100
⇒ a – b = 100 ...(i)
Now, a1000 of Ist A.P. = a + 999d
and a1000 of Ilnd A .P. = b + 999d
Hence, difference between their 1000th terms
= (a + 999d) (b + 999d) = a – b
= 100. [Using (i)]
The first term of an AP is 5, the last term is 45 and the sum is 400. Find the number of terms and the common difference.