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Đặt \(\frac{a}{b}=\frac{c}{d}=k\) thì \(a=bk,c=dk\).
\(\frac{2a+3b}{2a-3b}=\frac{2bk+3b}{2bk-3b}=\frac{b\left(2k+3\right)}{b\left(2k-3\right)}=\frac{2k+3}{2k-3}\\ \frac{2c+3d}{2c-3d}=\frac{2dk+3d}{2dk-3d}=\frac{d\left(2k+3\right)}{d\left(2k-3\right)}=\frac{2k+3}{2k-3}\)
Do đó: \(\frac{2a+3b}{2a-3b}=\frac{2c+3d}{2c-3d}\)
a)\(\frac{ab}{cd}=\frac{bk.b}{dk.b}=\frac{b^2}{d^2}\left(1\right)\)
\(\frac{a^2-b^2}{c^2-d^2}=\frac{b^2k^2-b^2}{d^2k^2-d^2}=\frac{b^2\left(k^2-1\right)}{d^2\left(k^2-1\right)}=\frac{b^2}{d^2}\left(2\right)\)
từ\(\left(1\right)\)và\(\left(2\right)\)\(\Rightarrow\frac{ab}{cd}=\frac{a^2-b^2}{c^2-d^2}\)
\(\frac{a^2+b^2}{c^2+d^2}=\frac{ab}{cd}.\)
\(\Rightarrow\left(a^2+b^2\right).cd=ab.\left(c^2+d^2\right)\)
\(\Rightarrow a^2cd+b^2cd=abc^2+abd^2\)
\(\Rightarrow a^2cd+b^2cd-abc^2-abd^2=0\)
\(\Rightarrow\left(a^2cd-abc^2\right)+\left(b^2cd-abd^2\right)=0\)
\(\Rightarrow ac.\left(ad-bc\right)+bd.\left(bc-ad\right)=0\)
\(\Rightarrow ac.\left(ad-bc\right)-bd.\left(ad-bc\right)=0\)
\(\Rightarrow\left(ad-bc\right).\left(ac-bd\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}ad-bc=0\\ac-bd=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}ad=bc\\ac=bd\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}\frac{a}{b}=\frac{c}{d}\left(đpcm\right).\\\frac{a}{b}=\frac{d}{c}\end{matrix}\right.\)
Vậy \(\frac{a}{b}=\frac{c}{d}.\)
Chúc bạn học tốt!
Cho tỉ lệ thức\(\frac{a}{b}=\frac{c}{d}\). Chứng minh rằng \(\frac{ab}{cd}=\frac{a^2-b^2}{c^2-d^2}\)
Đặt a/b=c/d=k
=> a=bk ; c=dk
Khi đó : a^2-b^2/c^2-d^2 = b^2k^2-b^2/d^2k^2-d^2 = b^2.(k^2-1)/d^2.(k^2-1) = b^2/d^2
Mà a/b=c/d => b/d = a/c => b^2/d^2 = a.b/c.d
=> a^2-b^2/c^2-d^2 = ab/cd
=> ĐPCM
Tk mk nha
a/ Đặt :
\(\dfrac{a}{b}=\dfrac{c}{d}=k\)\(\Leftrightarrow\left\{{}\begin{matrix}a=bk\\c=dk\end{matrix}\right.\)
Ta có :
\(VT=\dfrac{a-b}{a+b}=\dfrac{bk-b}{bk+b}=\dfrac{b\left(k-1\right)}{b\left(k+1\right)}=\dfrac{k-1}{k+1}\)\(\left(1\right)\)
\(VP=\dfrac{c-d}{c+d}=\dfrac{dk-d}{dk+d}=\dfrac{d\left(k-1\right)}{d\left(k+1\right)}=\dfrac{k-1}{k+1}\left(2\right)\)
Từ \(\left(1\right)+\left(2\right)\Leftrightarrowđpcm\)
b/ Đặt :
\(\dfrac{a}{b}=\dfrac{c}{d}=k\)\(\Leftrightarrow\left\{{}\begin{matrix}a=bk\\c=dk\end{matrix}\right.\)
\(VT=\dfrac{2a+5b}{3a-4b}=\dfrac{2bk+5b}{3bk-4b}=\dfrac{b\left(2k+5\right)}{b\left(3k-4\right)}=\dfrac{2k+5}{3k-4}\left(1\right)\)
\(VP=\dfrac{2c+5d}{3c-4d}=\dfrac{2dk+5d}{3dk-4d}=\dfrac{d\left(2k+5\right)}{d\left(3k-4\right)}=\dfrac{2k+5}{3k-4}\left(2\right)\)
Từ \(\left(1\right)+\left(2\right)\Leftrightarrowđpcm\)
a) Từ \(\dfrac{a}{b}=\dfrac{c}{d}\) \(\Rightarrow\dfrac{a}{c}=\dfrac{b}{d}\)
Áp dụng tính chất của dãy tỉ số bằng nhau, ta có:
\(\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{a+b}{c+d}=\dfrac{a-b}{c-d}\)
Từ \(\dfrac{a+b}{c+d}=\dfrac{a-b}{c-d}\) \(\Rightarrow\dfrac{c-d}{c+d}=\dfrac{a-b}{a+b}\)
b) Từ \(\dfrac{a}{b}=\dfrac{c}{d}\) \(\Rightarrow\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{2a}{2c}=\dfrac{3a}{3c}=\dfrac{4b}{4d}=\dfrac{5b}{5d}\)
Áp dụng tính chất của dãy tỉ số bằng nhau, ta có:
\(\dfrac{2a}{2c}=\dfrac{3a}{3c}=\dfrac{4b}{4d}=\dfrac{5b}{5d}=\dfrac{2a+5b}{2c+5d}=\dfrac{3a-4b}{3c-4d}\)
Từ \(\dfrac{2a+5b}{2c+5d}=\dfrac{3a-4b}{3c-4d}\) \(\Rightarrow\dfrac{2a+5b}{3a-4b}=\dfrac{2c+5d}{3c-4d}\)
Đặt k ta có
a/b = k => a=bk ; c/d= k => c= dk
Ta có : a2 + b2/ c2+d2 => bk2 + b2/ dk2+ d2 => [ b(k+1)]2/ [d(k+1)]2
vậy ab/cd = a2+b2/c2+d2
\(\dfrac{a}{b}=\dfrac{c}{d}\Rightarrow\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{a+b}{c+d}\Rightarrow\dfrac{a.b}{c.d}=\dfrac{a+b}{c+d}.\dfrac{a+b}{c+b}=\dfrac{\left(a+b\right)^2}{\left(c+d\right)^2}\)
Ta có:
\(\dfrac{a}{b}=\dfrac{c}{d}\Rightarrow\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{a+b}{c+d}\Rightarrow\dfrac{a.b}{c.d}=\dfrac{a+b}{c+d}.\dfrac{a+b}{c+d}=\dfrac{\left(a+b\right)^2}{\left(c+d\right)^2}\)
Ta có: \(\frac{a}{b}=\frac{c}{d}=k\)
\(\Rightarrow a=bk;c=dk\)
Lại có: \(\frac{ab}{cd}=\frac{bk.b}{dk.d}=\frac{kb^2}{kd^2}=\frac{b^2}{d^2}\)
Tương tự: \(\frac{a^2+b^2}{c^2+d^2}=\frac{k^2b^2+b^2}{k^2d^2+d^2}=\frac{b^2\left(k+1\right)}{d^2\left(k+1\right)}=\frac{b^2}{d^2}\)
=> đpcm
Lời giải:
Đặt $\frac{a}{b}=\frac{c}{d}=t\Rightarrow a=bt; c=dt$. Ta có:
$\frac{ab}{cd}=\frac{b^2t}{d^2t}=\frac{b^2}{d^2}(1)$
Mặt khác:
$\frac{(a-b)^2}{(c-d)^2}=\frac{(bt-b)^2}{(dt-d)^2}=\frac{b^2(t-1)^2}{d^2(t-1)^2}=\frac{b^2}{d^2}(2)$
Từ $(1); (2)\Rightarrow \frac{ab}{cd}=\frac{(a-b)^2}{(c-d)^2}$