Question

Find the sum of the series 57, 55, 53, …………….. 17.​

Answer 1

Answer:

174.25

Step-by-step explanation:

a=57

d=-2

n=17

Sn=n/2 [2a+(n-1)d]

=17/2 (114-32)

=17/2×82

=174.25

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Answer 2

Step-by-step explanation:

A1=57 ,A2 = 55, A3 = 53

d = A2-A1

D = 55 -57

d =-2

n=?

An = 17

An = A + (n-1) d

17 = 57 +(n-1) -2

17 -57=(n-1)-2

-40÷-2= (n-1)

20+1=n

21=n

Sn= n÷2(a+l)

Sn=21÷2 (57+ 17)

Sn=21÷2(60)

Sn=21×30

Sn=630

sum of the serios is 630 .

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