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Fie triunghiul ABC dreptunghic in A si AD inaltime .Aratati ca:
[tex] \frac{1}{ AD^{2}} = \frac{1}{ AB^{2} } + \frac{1}{ AC^{2} }[/tex]


Răspuns :

[tex] \frac{1}{AD ^{2} } = \frac{1}{AB ^{2} } + \frac{1}{AC ^{2} } [/tex] ⇔
[tex] \frac{1}{AD ^{2} } = \frac{AC ^{2} }{AC ^{2}*AB ^{2} } + \frac{AB ^{2} }{AB ^{2} *AC ^{2} } [/tex]⇔
[tex] \frac{1}{AD ^{2} } = \frac{AC ^{2} + AB ^{2} }{AC ^{2}*AB ^{2} }[/tex]⇔
[tex] \frac{1}{AD ^{2} } = \frac{BC ^{2} }{ 4*AriaABC }[/tex]⇔

4*AriaABC = BC² * AD² ⇔

4*AriaABC = 4*AriaABC - > Adevarat

Deci [tex] \frac{1}{AD ^{2} } = \frac{1}{AB ^{2} } + \frac{1}{AC ^{2} } [/tex]