If 10 times the 10th term of an A.P. is equal to 15 times the 15th term, show that the 25th term of the A.P. is Zero.
Given,
10 times the 10th term of an A.P. is equal to 15 times the 15th term
⇒ 10a10 = 15a15
To prove: a25 = 0
We know, an = a + (n – 1)d where a is first term or a1 and d is common difference and n is any natural number
When n = 10:
∴ a10 = a + (10 – 1)d
⇒ a10 = a + 9d
When n = 15:
∴ a15 = a + (15 – 1)d
⇒ a15 = a + 14d
When n = 25:
∴ a25 = a + (25 – 1)d
⇒ a25 = a + 24d ………(i)
According to question:
10a10 = 15a15
⇒ 10(a + 9d) = 15(a + 14d)
⇒ 10a + 90d = 15a + 210d
⇒ 10a – 15a + 90d – 210d = 0
⇒ -5a – 120d = 0
⇒ -5(a + 24d) = 0
⇒ a + 24d = 0
⇒ a25 = 0 (From (i))
Hence Proved