Question

The sum of the first 20 terms of the A.P. 10,6, 2…. is

a. -480 b. -400 c. -500 d. -560
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Answer 1

Answer:

We can rewrite the series as in the form

∑ {1/(n+4) - 1/(n+5)} , 20th term is 1/20+4 - 1/20+5 => 1/24 - 1/25

then the sum of first 20 terms of the series is

1/5 - 1/6 + 1/6 - 1/7 + 1/8 - 1/9 + 1/9 - 1/10 + … + 1/24 - 1/25

= 1/5 - 1/25 = 25–5 / 25*5 = 20 / 25*5 = 4/25

Answer 2

Answer:

A. - - -480

Step-by-step explanation:

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

Sn=10/2(2*10+(20-1)*4)

Sn=5(20+76)

Sn=5*96=480