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MedAndChemid

Răspuns:

[tex]\frac{a}{b}=\frac{c}{d}\\ad=cb\\\\(ad)^2=(cb)^2\\n=(a^2+b^2)*(c^2+d^2)=a^2*c^2+a^2*d^2+b^2*c^2+b^2*d^2=(ac)^2+(ad)^2+(bc)^2+(bd)^2=2(ad)^2+(ac)^2+(bd)^2=(ac+bd)^2 \\verificare \\(ac+bd)^2=(ac)^2+2acbd+(bd)^2=(ac)^2+2(ad)^2+(bd)^2[/tex]

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