Math Is Fun Forum

Alan556

Guest

Evaluate the sum

I'm having trouble with this question:

Evaluate this sum:

Thanks.

Identity

Member

Registered: 2007-04-18

Posts: 934

Re: Evaluate the sum

Try expanding it:

So the sum is 190-150-30=10

Although in this case, since the sum contains only 5 parts, you could quite easily do:

Last edited by Identity (2007-10-20 01:11:37)

Alan556

Guest

Re: Evaluate the sum

I went down the same line as this, but I couldn't see how to sum for r², and I can't really tell what you have done there. Would you mind explaining?

Thanks.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Evaluate the sum

He's using a fairly well-known identity:

.

As you want the sum from 4 to 8 but this formula has to start at 0, he's first worked out the sum from 0 to 8 and then taken away the sum from 0 to 3.

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Why did the vector cross the road?
It wanted to be normal.

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: Evaluate the sum

Here is a fast way to do it.

My advice: DON’T be too mechanical in applying your favourite formulas to every problem you meet. Always treat every problem as an individual case.