Question

construct difference of an ap is -2 and first term 80 .find the sum if last term is 10

Answer 1

HEY

HERE IS YOUR ANSWER,

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Answer:

             sum = 1620

Step-by-step explanation:

Given data :

Common difference => d = -2

First term => a = 80

last term => an = l = 10

By substituting in the formula

                       An = a + ( n - 1 ) d

10 = 80 + ( n -1 ) (-2)

[ multiplying -2 inside the bracket ]

10 = 80 - 2n + 2

bringing 2n to right hand side and constants to left hand side ,

2n = 80 + 2 + -10

2n = 70+2

2n = 72

n = 72/2

n = 36

By substituting the values in the formula ,

                          Sn = n/2 [ a + l  ]

Sn = 36/2 [ 80 + 10 ]

Sn = 18 [ 90 ]

Sn = 1620

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Answer 2

1620 is the ans....

hope its help....