Find the sum of the following number sequence: $2+4+6+8+10$ ?

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Answer 1 · 2016-04-22T06:39:20

30

We'll solve this bit by bit:
$2+4+6+8+10=6+6+8+10$
$=12+8+10$
$=20+10$
$=30$

Answer 2 · 2016-04-22T16:57:58

Sequences like this, where the numbers are spaced equally, have a quick way to sum up.

Add the first and the last term and divide by $2$ to get the mean. Then multiply by the number of terms.

$(2+10)/2xx5=12/2xx5=6xx5=30$

For long sequences this is much faster than just adding.
E.g. adding all numbers from $1->100$ inclusive:

$(1+100)/2xx100=101/cancel2xx(cancel2xx50)=5050$

Find the sum of the following number sequence: 2+4+6+8+102+4+6+8+10?