the In an arithmetic Sequence, Sum of first 9 terms is 279 and the sum of first 20 terms is 1280, th

Question

the
In
an
arithmetic Sequence, Sum of
first 9 terms is
279 and the sum of first
20 terms is
1280,
then,
a) what is the 5th term of this sequence?
b)what is 16th term of the
Sequence ?

c) write
down
the sequence ? n²+n​

in progress 0
Eloise 5 months 2022-01-06T02:37:33+00:00 1 Answer 0 views 0

Answers ( )

  1. Charlie
    0
    2022-01-06T02:39:13+00:00
    Reply

    Answer:

    dm on @__princely___ for algebra doubts

    Step-by-step explanation:

    Sn = n ( a1 + an ) / 2

    In this case:

    S9 = 9 ( a1 + a9 ) / 2 = 279

    S20 = 20 ( a1 + a20 ) / 2 = 1280

    9 ( a1 + a9 ) / 2 = 279 Multiply both sides by 2

    9 ( a1 + a9 ) = 279 * 2

    9 a1 + 9 a9 = 558

    On the other side:

    an = a1 ( n – 1 ) d

    in this case: n = 9, n – 1 = 8

    a9 = a1 + 8 d

    9 a1 + 9 a9 = 558

    9 a1 + 9 ( a1 + 8 d ) = 558

    9 a1 + 9 a1 + 9 * 8 d = 558

    9 a1 + 9 a1 + 72 d = 558

    18 a1 + 72 d = 558

    20 ( a1 + a20 ) / 2 = 1280 Multiply both sides by 2

    20 ( a1 + a20 ) = 1280 * 2

    20 a1 + 20 a20 = 2560

    an = a1 ( n – 1 ) d

    In this case: n = 20, n – 1 = 19

    a20 = 20 a1 + 19 d

    20 a1 + 20 a20 = 2560

    20 a1 + 20 ( a1 + 19 d ) = 2560

    20 a1 + 20 a1 + 20 * 19 d = 2560

    20 a1 + 20 a1 + 380 d = 2560

    40 a1 + 380 d = 2560

    Now you must solve system of 2 equations with 2 unknows:

    18 a1 + 72 d = 558

    40 a1 + 380 d = 2560

    The solutions are :

    a1 = 7

    d = 6

    an = a1 + ( n – 1 ) d

    a5 = 7 + ( 5 – 1 ) * 6

    a5 = 7 + 4 * 6

    a5 = 7 + 24

    a5 = 31

    an = a1 + ( n – 1 ) d

    a16 = 7 + ( 16 – 1 ) * 6

    a16 = 7 + 15 * 6

    a16 = 7 + 90

    a16 = 97

    Your sequence:

    7, 13, 19 , 25, 31, 37, 43, 49, 55, 61, 67, 73, 79, 85, 91, 97, 103, 109, 115, 121

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