Find the sum of the first twenty terms of 5, 11, 17, 23, ...?

Question

Find the sum of the first twenty terms of 5, 11, 17, 23, ...

a. 1300
b. 1190
c. 1240
d. 2480

Answer 1

A little simple logic in lieu of mysterious and abstract formulas:
Notice each term in the sequence is 6 more than the last (except for the first term, of course).
so we're summing up
5 +
5 + 6 +
5 + 6 + 6 +
5 + 6 + 6 + 6 + ...
There are twenty 5's (20 × 5).
We're left with (6) + (6+6) + (6+6+6) + ...
i.e., (6) + (2×6) + (3×6) + ...
You can factor out the 6:
6 × (1 + 2 + 3 + ... + 19)
And rearrange the terms:
6 × (1+19 + 2+18 + ... + 11+9 + 10)
and see that each pair of terms is 20 with an extra 10.
There are nine of these pairs, so you have:
6 × 9 × (20) + 10.
no, not really...
it's 6 × (9 × 20 + 10), so

Add to the twenty 5's:
20×5 + 6 × (9×20+10)
and you have 1240.

Feel free to email me for further clarifications.

Answer 2

Sn=n/2[2a+(n-1)d]
=20/2[10+19*6]
=1240

Answer 3

5+11+17+23+29+35+41+47+53+59+65+71+77+83+89+95+101+107+113+119=1240

The answer is C

Answer 4

1240

Answer 5

Bugman - I suspect your error is here with the order of calculations

Add to the twenty 5's:
20×5 + 6×9×20 + 10
and you have 1190.

I guess you've done (6*9*20) +10
You should have done 6*((9*20) + 10)

Answer 6

5+11+17+23+…=5+(5*2+1)+(5*3+2)+(5*4+3)+…+(5*20+19)
(last nombur=19*6+5)
=5(1+2+3+4+…+20)+(1+2+3+4+…+19)
=5((n*n+1)/2)+((n'*n'+1)/2)
= 5((n*n+1)/2)+((n'*n'+1)/2) =5*210+190=1240

[ n =20 : ((n*n+1)/2)=(20*21)/2=210 ]
[n'=19 : ((n'*n'+1)/2)=(19*20)/2=190 ]


Item C is correct.

Answer 7

one million) 3 words to describe the colour WHITE empty, description, call 2) you're surrounded by WATER - 3 words to describe island, separation, independence 3) 1st coloration to bypass into your head - then 3 words to describe it pink: warmth, passionate, suspended (in time) 4) 1st ANIMAL to bypass into your head - then 3 words to describe it canines: Loving, danger unfastened, non-judgmental 5) you're surrounded by woodland - 3 words to describe it residing house, attractiveness, philosophical

Answer 8

Sn = (a1 + an)*n/2
Sn = (5 + 119)*10
Sn = 124*10
Sn = 1240
Answer is c).
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Answer 9

1,240

Answer 10

C