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by sudhir3127 » Thu Aug 07, 2008 8:57 am
Find the value of

1-2-3 + 2-3-4+.........+ 100 terms

Answer after some discussion...
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Source: — Problem Solving |
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by pepeprepa » Thu Aug 07, 2008 9:01 am
I do not see how the set finishes...
is the end 98-99-100
or 98-99-100+100
or there are 100 terms in the set ?
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by sudhir3127 » Thu Aug 07, 2008 9:03 am
it says upto 100 terms....
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by nitingupta5 » Thu Aug 07, 2008 9:06 am
Is it -626
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by pepeprepa » Thu Aug 07, 2008 9:13 am
Yeah I agree with -626
The set goes from
1-2-3
to
33-34-35
34
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by sudhir3127 » Thu Aug 07, 2008 9:20 am
thanks for the answer...

its -626...

here goes the answer..

1-2-3 + 2-3-4+............+upto 100 terms

-4 + -5 ................33 terms + 34

n= 33

n/2 ( 2a + (n-1 ) d) + 34

33/2 ( 2(-4) + 32(-1) + 34

-660 + 34 = -626

thought it to be a bit tricky.. hence shared...

thanks a lot all..
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by pepeprepa » Thu Aug 07, 2008 9:29 am
Indeed, I do not understand which technique you use?

I used the sum formula:
1-2-3
2-3-4
...
33-34-35
34

T=( "The sum from i=1 to 33" of (i-(i+1)-(i+2)) ) + 34
T=("The sum from i=1 to 33" of (-i-3) ) + 34
T=( 33*(-3) - "The sum from i=1 to 33" of i ) +34
T= (-99 - 33*34/2) + 34
T= -660 + 34
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by nitingupta5 » Thu Aug 07, 2008 9:36 am
What Sudhir did is added up the first three terms and came up with -4
and the same with the rest

-4, -5,...

It's a faster way to do it

I did the same way as you describe
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