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Elemente de algebra liniara.pdf

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Conice 123<br />

iar parametrii<br />

<br />

<br />

<br />

<br />

<br />

<br />

a11 a12 a13 <br />

a11 a12 <br />

<br />

<br />

I = a11 + a22, δ = , ∆ = a12 a22 a23 <br />

a12 a22 <br />

<br />

a13 a23 a33 <br />

rămân invariant¸i, adică a11 + a22 = a ′ 11 + a′ 22 ,<br />

<br />

<br />

a11 a12 <br />

<br />

a12 a22 =<br />

<br />

<br />

<br />

<br />

<br />

Demonstrat¸ie. Relat¸ia<br />

a ′ 11 a′ 12<br />

a ′ 12 a′ 22<br />

<br />

<br />

<br />

<br />

,<br />

<br />

<br />

<br />

<br />

a11 a12 a13 <br />

<br />

<br />

a12 a22 a23 = <br />

<br />

<br />

a13 a23 a33 <br />

a ′ 11 a′ 12 a′ 13<br />

a ′ 12 a′ 22 a′ 23<br />

a ′ 13 a′ 23 a′ 33<br />

f(x, y) = a11 x 2 + 2 a12 xy + a22 y 2 + 2a13 x + 2a23 y + a33<br />

se poate scrie în formă matriceală<br />

⎛<br />

⎜<br />

f(x, y) = ( x y 1 ) ⎝<br />

utilizând notat¸iile<br />

X =<br />

<br />

x<br />

y<br />

<br />

Relat¸ia <br />

se poate scrie sub forma<br />

⎛<br />

⎜<br />

⎝<br />

x<br />

y<br />

1<br />

a11 a12 a13<br />

a12 a22 a23<br />

a13 a23 a33<br />

, B =<br />

⎞<br />

⎟<br />

⎠ =<br />

sau <br />

<br />

⎞ ⎛<br />

⎟ ⎜<br />

⎠ ⎝<br />

a13<br />

a23<br />

<br />

x<br />

y<br />

1<br />

⎞<br />

⎟<br />

⎠ = ( t X 1 )<br />

, A =<br />

x = x0 + x ′ cos α − y ′ sin α<br />

y = y0 + x ′ sin α + y ′ cos α<br />

⎛<br />

⎜<br />

⎝<br />

X<br />

1<br />

cos α − sin α x0<br />

sin α cos α y0<br />

0 0 1<br />

<br />

=<br />

<br />

R T<br />

0 1<br />

<br />

<br />

⎞ ⎛<br />

⎟ ⎜<br />

⎠ ⎝<br />

X ′<br />

1<br />

<br />

<br />

a11 a12<br />

a12 a22<br />

x ′<br />

y ′<br />

1<br />

⎞<br />

⎟<br />

⎠<br />

A B<br />

t B a33<br />

<br />

.<br />

<br />

<br />

<br />

<br />

<br />

.<br />

<br />

<br />

X<br />

1

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