Alberto Monteiro wrote:
>
> Ronn!Blankenship wrote:
> >
> >> Who somehow did memorize the quadratic formula...
> >
> > Can you derive it?
> >
> Trivial. I ddn't memorize Cardano's formula, but I can
> derive it easily: eliminate term in x^2, x = u + v then
> eliminate term with uv.
He did say quadratic, which means second degree?
> OTOH, I have a hard time remembering some obscure
> geometry formulas, even simple ones like
> a^2 = b^2 + c^2 - [or +?] 2 [?] b c cos [or sin?] A
Yes, I tend to derive the Law of Cosines from the
dot product of vectors when I need it. (Although once you
have it down to + or - sin or cos, it's easy to decide
which of the 4 possibilities is correct. The last term
must be zero when the angle is 90 degrees, and so on...)
---David
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