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Elemente de algebra liniara.pdf

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24 <strong>Elemente</strong> <strong>de</strong> algebră liniară<br />

<strong>Elemente</strong>le lui V se numesc vectori iar elementele lui K se numesc scalari. Un spat¸iu<br />

vectorial peste R este numit spatiu vectorial real iar un spat¸iu vectorial peste C este<br />

numit spat¸iu vectorial complex. In loc <strong>de</strong> x + (−y) scriem x − y.<br />

Propozit¸ia 2.2 Dacă V este un spat¸iu vectorial atunci:<br />

a) αx = 0 ⇐⇒ α = 0 sau x = 0<br />

b) α(−x) = (−α)x = −αx<br />

c) α(x − y) = αx − αy<br />

d) (α − β)x = αx − βx.<br />

Demonstrat¸ie. a)<br />

b)<br />

c)<br />

d)<br />

αx = 0<br />

α = 0<br />

<br />

=⇒ x = 1<br />

0 = 0<br />

α<br />

0x = (0 + 0)x = 0x + 0x =⇒ 0x = 0<br />

α(0 + 0) = α0 + α0 =⇒ α0 = 0.<br />

0 = α0 = α(x + (−x)) = αx + α(−x) =⇒ α(−x) = −αx<br />

0 = 0x = (α + (−α))x = αx + (−α)x =⇒ (−α)x = −αx.<br />

α(x − y) = α(x + (−y)) = αx + α(−y) = αx − αy.<br />

(α − β)x = (α + (−β))x = αx + (−β)x = αx − βx.<br />

Exemplul 2.1 (R 3 , +, ·), un<strong>de</strong><br />

este un spat¸iu vectorial real.<br />

Exemplul 2.2 Mult¸imea<br />

(x1, x2, x3) + (y1, y2, y3) = (x1 + y1, x2 + y2, x3 + y3)<br />

α(x1, x2, x3) = (αx1, αx2, αx3)<br />

M2×3(R) =<br />

<br />

x11 x12 x13<br />

x21 x22 x23<br />

xij ∈ R

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