5
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Place any three numbers on the vertices of a triangle:

         5
        / \
       /   \
      /     \
     8-------4

Calculate their differences, going one way:

         5
        / \
   -3  /   \ -1
      /     \
     8-------4
         4

Place another number inside the triangle:

         5
        / \
   -3  /   \ -1
      / 15  \
     8-------4
         4

Add the centre number to a vertex, and multiply the total by the opposite edge. Do this for each vertex. Add them together.

$(15+5)\times4=80$
$(15+8)\times-1=-23$
$(15+4)\times-3=-57$

$80-23-57=0$

You got $0$. Why?

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1
  • 2
    $\begingroup$ Because math is cool. $\endgroup$ Apr 7 '20 at 11:41
7
$\begingroup$

Although weird-looking in hindsight, some elementary maths clearly show why.
fig

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2
  • $\begingroup$ Should be $(d+c)(a-b)$ et cetera $\endgroup$ Apr 7 '20 at 11:45
  • $\begingroup$ Sorry, totally forgot about that when I started copying that formula :( Fixed, thanks! @DanielMathias $\endgroup$
    – newbie
    Apr 7 '20 at 11:48

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