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If I have a right triangle, of the form: $ABC$, with right angle at B:
Then, $BM = CM = AM$.
Then, as I show that:
$BM = AM$
The simplest demonstration is to create another triangle equal to this one, and adjust them so that it looks like a rectangle. The diagonals of a rectangle are the same, which shows the property.
But I would like to see the demonstration, without using the property of diagonals of a rectangle, in advance thanks.
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$\begingroup$Drop the altitude from $M$ to $AB$ which will be parallel to $BC$ and find two equal right triangles.
$\endgroup$ 5 $\begingroup$Consider the circumcircle of ABC. Since ABC=90º, AC is the diameter.
Thus M is the circumcenter and MC=MB=MA.
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