Cho a/b=c/d.CMR a)5a+3c/5b+3d=5a-3c/5b-3d b)5a+3b/5c+3d=5a-3b/5c-3d c)(a+b/c+d)^3=a^3-b^3/c^3-d^3 GIÚP EM NHA..EM CẢM ƠN

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1. Giải thích các bước giải:

   sử dụng $\dfrac{a}{b}= \dfrac{c}{d}= \dfrac{a-c}{b-d}= \dfrac{a+c}{b+d}$ 

   a) $\dfrac{a}{b}= \dfrac{c}{d}$
   $\Rightarrow \dfrac{5a}{5b}= \dfrac{3c}{3d}$
   $\Rightarrow \dfrac{5a-3c}{5b-3d}= \dfrac{5a}{5b}= \dfrac{3c}{3d}= \dfrac{5a+3c}{5b+3d}$

   b) $\dfrac{a}{b}= \dfrac{c}{d}\Rightarrow \dfrac{a}{c}= \dfrac{b}{d}$
   $\Rightarrow \dfrac{5a}{5c}= \dfrac{3b}{3d}$
   $\Rightarrow \dfrac{5a-3b}{5c-3d}= \dfrac{5a}{5c}= \dfrac{3b}{3d}= \dfrac{5a+3b}{5c+3d}$

   c) $\dfrac{a}{b}= \dfrac{c}{d}\Rightarrow \dfrac{a}{c}= \dfrac{b}{d}$
   $\Rightarrow \dfrac{a}{c}= \dfrac{b}{d}= \dfrac{a+b}{c+d}$
   $\Rightarrow \dfrac{a^{3}}{c^{3}}= \dfrac{b^{3}}{d^{3}}= \dfrac{\left ( a+b \right )^{3}}{\left ( c+d \right )^{3}}= \dfrac{a^{3}-b^{3}}{c^{3}-d^{3}}$

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