Given:
Sum of first 7 terms of an AP = 49.
Sum of first 17 terms = 289
Find:
the sum of first 30 term
Solution:
We know that,
Sum of first n terms of an AP – S(n) = n/2 * [ 2a + (n - 1)d ]
Hence,
→ 7/2 * [ 2a + (7 - 1)d ] = 49
→ 2a + 6d = 49 * 2/7
→ 2( a + 3d ) = 14
→ a + 3d = 14/2
→ a + 3d = 7 -- equation (1)
Similarly,
→ 17/2 * [ 2a + 16d ] = 289
→ 17/2 * 2 * (a + 8d) = 289
→ a + 8d = 289 * 2/17 * 1/2
→ a + 8d = 17 -- equation (2)
Subtract equation (1) from (2).
→ a + 8d - (a + 3d) = 17 - 7
→ a + 8d - a - 3d = 10
→ 5d = 10
→ d = 10/5
→ d = 2
Substitute the value of d in equation (1).
→ a + 3 * 2 = 7
→ a = 7 - 6
→ a = 1
Now,
Sum of first 30 terms = 30/2 * [ 2(1) + (30 - 1)(2) ]
→ S(30) = 15 (2 + 58)
→ S(30) = 15 * 60
→ S(30) = 900
Therefore, the sum of first 30 terms of the given AP is 900.
I hope it will help you.
Regards.