[Đại số 8] Rút gọn

E

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$P=\dfrac{(a+c)(a+d)(b+c)(b+d)}{(a+b+c+d)^2}$

$=\dfrac{(a^2+ad+ac+cd)(b^2+bd+bc+cd)}{(a+b+c+d)^2}$

$=\dfrac{(a^2+ad+ac+ab)(b^2+bd+bc+ab)}{(a+b+c+d)^2}$

$=\dfrac{ab(a+b+c+d)^2}{(a+b+c+d)^2}=ab$
 
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