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# Cho dãy tỉ số bằng nhau ( 2bz-3cy )/a=(3cx az)/2b=(ay-2bx)/3c. Chứng minh: x/a=y/2b=z/3c.
Trả lời
Đáp án:+Giải thích các bước giải:
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Theo đề: \(\frac{2bz-3cy}{a}=\frac{3cx-az}{2b}=\frac{ay-2bx}{3c}\)
\(\Rightarrow\frac{2bza-3acy}{a^2}=\frac{6cxb-2bza}{4b^2}=\frac{3ayc-6bxc}{9c^2}\)
\(=\frac{2bza-3cya+6xbc-2bza+3ayc-6bxc}{a^2+4b^2+9c^2}\)
\(=0\)
\(\Rightarrow\frac{2bz-3cy}{a}=\frac{3cx-az}{2b}=\frac{ay-2bx}{3c}=0\)
\(\Rightarrow2bz=3cy;3cx=az;ay=2bx\)
\(\Rightarrow\frac{x}{a}=\frac{y}{2b}=\frac{z}{3c}\left(đpcm\right)\)
Ta có: \(\frac{2bz-3cy}{a}=\frac{3cx-az}{2b}=\frac{ay-2bx}{3c}\)
\(\Rightarrow\frac{2bzx-3cyx}{ax}=\frac{3cxy-azy}{2by}=\frac{ayz-2bxz}{3xz}\)
\(=\frac{2bzx-3cyx-3cxy-azy-ayz-2bxz}{ax-2by-3xz}=0\)
\(\Rightarrow\)\(\frac{2bz-3cy}{a}=\frac{3cx-az}{2b}=\frac{ay-2bx}{3c}=0\)
\(\Rightarrow2bz=3cy;\)\(3cx=az;\)\(ay=2bx\)
\(\Rightarrow\frac{x}{a}=\frac{y}{2b}=\frac{z}{3c}\).
\(\dfrac{2bz-3cy}{a}=\dfrac{3cx-az}{2b}=\dfrac{ay-2bx}{3c}\\ \Rightarrow\dfrac{2abz-3acy}{a}=\dfrac{6bcx-2abz}{2b}=\dfrac{3acy-6bcx}{3c}\\ =\dfrac{\left(2abz-3acy\right)+\left(6bcx-2abz\right)+\left(3acy-6bcx\right)}{a+2b+3c}\\ =\dfrac{\left(2abz-2abz\right)+\left(3acy-3acy\right)+\left(6bcx-6bcx\right)}{a+2b+3c}=0\\ \)
\(\Rightarrow2bz-3cy=3cx-az=ay-2bx=0\\ \Rightarrow\left\{{}\begin{matrix}2bz=3cy\\3cx=az\\ay=2bx\end{matrix}\right.\)
\(2bz=3cy\Rightarrow\dfrac{2b}{y}=\dfrac{3c}{z}\\ 3cx=az\Rightarrow\dfrac{3c}{z}=\dfrac{a}{x}\\ ay=2bx\Rightarrow\dfrac{a}{x}=\dfrac{2b}{y}\\ \Rightarrow\dfrac{a}{x}=\dfrac{2b}{y}=\dfrac{3c}{z}\Rightarrow.....\)
\(\frac{2bz-3cy}{a}=\frac{3cx-az}{2b}=\frac{ay-2bx}{3c}\)
Suy ra: \(\frac{a.\left(2bz-3cy\right)}{a.a}=\frac{2b\left(3cx-az\right)}{2b.2b}=\frac{3c.\left(ay-2bx\right)}{3c.3c}\)
\(\Rightarrow\frac{2abz-3acy}{a^2}=\frac{3bcx-abz}{2b^2}=\frac{acy-2cbx}{3c^2}\)
Theo tính chất dãy tỉ số bằng nhau
\(\frac{2abz-3acy+6bcx-2abz+3acy-6bcx}{a^2+2b^2+3c^2}=\frac{0}{a^2+2b^2+3c^2}=0\)
\(\Rightarrow\hept{\begin{cases}2bz=3cy\\3cx=az\\ay=2bx\end{cases}\Rightarrow\hept{\begin{cases}\frac{z}{3c}=\frac{y}{2b}\\\frac{x}{a}=\frac{z}{3c}\\\frac{y}{2b}=\frac{x}{a}\end{cases}}\Rightarrow\frac{x}{a}=\frac{y}{2b}=\frac{z}{3c}}\)
=> đpcm