Ta có: \(\dfrac{a}{b}=\dfrac{c}{d}\)

nên \(\dfrac{5a}{3b}=\dfrac{5c}{3d}\)

hay \(\dfrac{5a}{5c}=\dfrac{3b}{3d}\)

Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:

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

\(\Leftrightarrow\dfrac{5a+3b}{5c+3d}=\dfrac{5a-3b}{5c-3d}\)

hay \(\dfrac{5a+3n}{5a-3b}=\dfrac{5c+3d}{5c-3d}\)(đpcm)

cho \(\frac{a}{b}\)=\(\frac{c}{d}\)=k=> a=bk; c=dk

a. Vế trái =\(\frac{5a+3b}{5a-3b}\)=\(\frac{5bk+3b}{5bk-3b}\)=\(\frac{b\left(5k+3\right)}{b\left(5k-3\right)}\)=\(\frac{\left(5k+3\right)}{\left(5k-3\right)}\)(1)

Vế phải =\(\frac{5c+3d}{5c-3d}\)=\(\frac{5dk+3d}{5dk-3d}\)=\(\frac{d\left(5k+3\right)}{d\left(5k-3\right)}\)=\(\frac{\left(5k+3\right)}{\left(5k-3\right)}\)(2)

Từ (1) và (2) ta có\(\frac{5a+3b}{5a-3b}\)=\(\frac{5c+3d}{5c-3d}\)

b. Vế trái=\(\frac{7a^2+3ab}{11a^2-8b^2}\)=\(\frac{7b^2k^2+3b.k.b}{11b^2.k^2-8b^2}\)=\(\frac{b^2.k\left(7k+3\right)}{b^2\left(11k^2-8\right)}\)=\(\frac{k\left(7k+3\right)}{\left(11k^2-8\right)}\)(1)

Vế phải =\(\frac{7c^2+3cd}{11c^2-8d^2}\)=\(\frac{7d^2k^2+3d.k.d}{11d^2.k^2-8d^2}\)=\(\frac{d^2.k\left(7k+3\right)}{d^2\left(11k^2-8\right)}\)=\(\frac{k\left(7k+3\right)}{\left(11k^2-8\right)}\)(2)

Từ (1) và (2) ta có: \(\frac{7a^2+3ab}{11a^2-8b^2}\)=\(\frac{7c^2+3cd}{11c^2-8d^2}\)

giups mình với cảm ơn

Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\Rightarrow a=bk,c=dk\)

a) \(\dfrac{a^2-b^2}{c^2-d^2}=\dfrac{b^2k^2-b^2}{d^2k^2-d^2}=\dfrac{b^2}{d^2}\)\(=\dfrac{\dfrac{a}{k}.b}{\dfrac{c}{k}.d}=\dfrac{ab}{cd}=VT\)

Vậy...

b) \(\dfrac{5a+3b}{5a-3b}=\dfrac{5bk+3b}{5bk-3b}=\dfrac{5k+3}{5k-3}\)

\(\dfrac{5c+3d}{5c-3d}=\dfrac{5dk+3d}{5dk-3d}=\dfrac{5k+3}{5k-3}\)

Suy ra \(\dfrac{5a+3b}{5a-3b}=\dfrac{5c+3d}{5c-3d}\)

c) \(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7\left(bk\right)^2+3\left(bk\right).b}{11\left(bk\right)^2-8b^2}\)\(=\dfrac{7k^2+3k}{11k^2-8}\)

\(\dfrac{7c^2+3cd}{11c^2-8d^2}=\dfrac{7\left(dk\right)^2+3\left(dk\right).d}{11\left(dk\right)^2-8d^2}=\dfrac{7k^2+3k}{11k^2-8}\)

Suy ra \(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}\)

a) Có: \(\dfrac{a}{b}=\dfrac{c}{d}\)

=> \(ad=bc\)

=> \(\dfrac{a}{c}=\dfrac{b}{d}\) => \(\left(\dfrac{a}{c}\right)^2=\left(\dfrac{b}{d}\right)^2=\dfrac{ab}{cd}=\dfrac{a^2}{c^2}=\dfrac{b^2}{d^2}=\dfrac{a^2-b^2}{c^2-d^2}\)

(theo tính chất dãy tỉ số bằng nhau)

=> (đpcm)

b) Có: \(\dfrac{a}{b}=\dfrac{c}{d}\) => \(\dfrac{a}{c}=\dfrac{b}{d}\)

=> \(\dfrac{5a}{5c}=\dfrac{3b}{3d}=\dfrac{5a+3b}{5c+3d}=\dfrac{5a-3b}{5c-3d}\)(theo tính chất dãy tỉ số bằng nhau)

=> \(\dfrac{5a+3b}{5a-3b}=\dfrac{5c+3d}{5c-3d}\) (đpcm)

c) Có: \(\dfrac{a}{b}=\dfrac{c}{d}\Leftrightarrow\dfrac{a}{c}=\dfrac{b}{d}\)

=> \(\dfrac{a^2}{c^2}=\dfrac{ab}{cd}=\dfrac{b^2}{d^2}\)          => \(\dfrac{7a^2}{7c^2}=\dfrac{3ab}{3cd}=\dfrac{11a^2}{11c^2}=\dfrac{8b^2}{8d^2}\)

=> \(\dfrac{7a^2+3ab}{7c^2+3cd}=\dfrac{11a^2-8b^2}{11c^2-8d^2}\) (theo tính chất dãy tỉ số bằng nhau)

=> \(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}\)(đpcm)

#Ayumu

tra mạng có bài giải chi tiết rồi đó bạn

cảm ơn

đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\)\(\Rightarrow\left\{{}\begin{matrix}a=bk\\c=dk\end{matrix}\right.\)

a) thay \(a=bk;c=dk\) ta có

\(\dfrac{5a+3b}{5a-3b}=\dfrac{5bk+3b}{5bk-3b}=\dfrac{b\left(5k+3\right)}{b\left(5k-3\right)}=\dfrac{5k+3}{5k-3}\)(1)

\(\dfrac{5c+3d}{5c-3d}=\dfrac{5dk+3d}{5dk-3d}=\dfrac{d\left(5k+3\right)}{d\left(5k-3\right)}=\dfrac{5k+3}{5k-3}\)(2)

từ (1);(2)\(\Rightarrow\dfrac{5a+3b}{5a-3b}=\dfrac{5c+3d}{5c-3d}\)

b) thay \(a=bk;c=dk\) ta có

\(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7(bk)^2+3bkb}{11(bk)^2-8b^2}=\dfrac{7b^2k^2+3b^2k}{11b^2k^2-8b^2}\)

\(=\dfrac{b^2\left(7k^2+3k\right)}{b^2\left(11k^2-8\right)}=\dfrac{7k^2+3k}{11k^2-8}\)(3)

\(\dfrac{7c^2+3cd}{11c^2-8d^2}=\dfrac{7\left(dk\right)^2+3dkd}{11\left(dk\right)^2-8d^2}=\dfrac{7d^2k^2+3d^2k}{11d^2k^2-8d^2}\)

\(=\dfrac{d^2\left(7k^2+3k\right)}{d^2\left(11k^2-8\right)}=\dfrac{7k^2+3k}{11k^2-8}\)(4)

từ (3);(4)\(\Rightarrow\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}\)

à quên 
\(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{a}{c}=\frac{b}{d}\Rightarrow\frac{5a}{5c}=\frac{3b}{3d}\)
Áp dụng tính chất dãy tỉ số = nhau 
\(\frac{5a}{5c}=\frac{3b}{3d}=\frac{5a+3b}{5c+3d}=\frac{5a-3b}{5c-3d}\)
\(\Rightarrow\frac{5a+3b}{5a-3b}=\frac{5c+3d}{5c-3d}\)

LÀM CÁCH GIẢI LUÔN ĐI!!~~
 

Đặt a/b=c/d=k

=>a=bk; c=dk

a: \(\dfrac{5a+3b}{5a-3b}=\dfrac{5bk+3b}{5bk-3b}=\dfrac{5k+3}{5k-3}\)

\(\dfrac{5c+3d}{5c-3d}=\dfrac{5dk+3d}{5dk-3d}=\dfrac{5k+3}{5k-3}\)

Do đó: \(\dfrac{5a+3b}{5a-3b}=\dfrac{5c+3d}{5c-3d}\)

b: \(\dfrac{7a^2+8ab}{11a^2-8b^2}=\dfrac{7b^2k^2+8\cdot bk\cdot b}{11\cdot b^2\cdot k^2-8b^2}=\dfrac{7k^2+8k}{11k^2-8}\)

\(\dfrac{7c^2+3cd}{11c^2-8d^2}=\dfrac{7d^2k^2+8\cdot dk\cdot d}{11\cdot d^2\cdot k^2-8d^2}=\dfrac{7k^2+8k}{11k^2-8}\)

Do đó: \(\dfrac{7a^2+8ab}{11a^2-8b^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}\)