find the sum of first 20+40–.800​

Question

find the sum of first 20+40………800​

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Ayla 8 months 2021-10-03T09:14:23+00:00 2 Answers 0 views 0

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  1. Eliza
    0
    2021-10-03T09:15:45+00:00
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    Step-by-step explanation:

    ans:-sum of terms=16400

  2. Eliza
    0
    2021-10-03T09:16:21+00:00
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    Answer:

    Heya☺️✌️♥️

    Step-by-step explanation:

    ✏️Solution :-

    Let a be the first term and d be the common

    difference of the given A.P.

    And the sum of the first 20 terms be S(20).

    S(20) = 20/2[2a + 19d]

    or, 400 = 20/2[2a + 19d]

    or, 400 = 10[2a + 19d]

    or, 2a + 19d = 40 ….. (i)

    Also, S(40) = 40/2[2a + 39d]

    or, 1600 = 20[2a + 39d]

    or, 2a + 39d = 80 ….(ii)

    From (i) and (ii), we get

    2a + 39d = 40

    2a + 19d = 80

    ___________

    –     –         –

    ⇒ 20d = 40

    ⇒ d = 40/20

    ⇒ d = 2

    Putting d’s value in Eq (i), we get

    ⇒ 2a + 19d = 40

    ⇒ 2a + 19(2) = 40

    ⇒ 2a + 38 = 40

    ⇒ 2a = 40 – 38

    ⇒ 2a = 2

    ⇒ a = 2/2

    ⇒ a = 1

    Then, S(10) = [2 × 1 + (10 – 1)2]

    ⇒ S(10) = 5[2 + 9 × 2]

    ⇒ S(10) = 5[2 + 18]

    ⇒ S(10) = 5 × 20

    ⇒ S(10) = 100

    Hence, the sum of its first 10 terms is 100…

    ✒️Hope it works♥️✌️☺️...

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