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A B C D E is pentagon, prove that v...

`A B C D E` is pentagon, prove that ` vec A B` + ` vec B C` + ` vec C D` + ` vec D E+ vec E A` = ` vec0` ` vec A B+ vec A E+ vec B C+ vec D C+ vec E D+ vec A C=3 vec A C`

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`vecR=vec(AB)+vec(AE)+vec(BC)+vec(DC)+vec(ED)+vec(AC)`
`" "=(vec(AB)+vec(B C))+(vec(AE)+vec(ED)+vec(DC))+vec(AC)`
`" "=vec(AC)+vec(AC)+vec(AC)=3vec(AC)`
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