Question

in the arthematic progression the 18th and 8yh term is 20 and be first term is 3. find the first 10 terms​

Answer 1

Answer:

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Answer 2

Answer:

Step-by-step explanation:

Given that,

18th term of an A.P. = a 18 = a + 17d = 20 and

8th term of an A. P. = a 8 = a + 6d = 20 and

First term of an A. P. = a 1 = a = 3

Now we find " d "

So, put " a " value in 8th term ( a +6d = 20 ),we get

3 + 6d = 20 ⇒ 6d = 20 - 3 ⇒ 6d = 17 ⇒ d = 17 / 6

Thus we have a = 3 and d = 17 /6

So,Sum of first n terms Sn = n / 2 [ 2a + ( n - 1 ) d ]

∴ Sum of first 10 terms S10 = 10 / 2 [ 2 × 3 + ( 10 - 1 ) 17/6 ]

= 5 [ 6 + 9 × 17/6 ] = 5 ( 6 + 3 × 17 / 2 ) ( ∵ 3 times by 9 and 2 times by 6 are cancel)

= 5 ( 6 + 51 / 2 ) = 5 ( 6 ×2 + 51 / 2 ) = 5 ( 12 + 51 / 2 ) = 5 ( 63 / 2 ) = 315 / 2