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Áp dụng tính chất của dãy tỉ số = nhau ta có:
\(\frac{a}{b}=\frac{b}{c}=\frac{c}{d}=\frac{d}{a}=\frac{a+b+c+d}{b+c+d+a}=1\)
=> a = b = c = d
=> \(D=\frac{2a-a}{2a-a}+\frac{2a-a}{2a-a}+\frac{2a-a}{2a-a}+\frac{2a-a}{2a-a}\)
D = 1 + 1 + 1 + 1 = 4
Ta có : 2a + b + c+ d / a - 1 = a + 2b + c + d / b - 1 = a + b + 2c + d / c - 1 = a + b + c +2d / d - 1
=> a + b + c + d / a = a + b + c + d / b = a + b + c + d / c = a + b + c + d / d
Xét 2 trường hợp :
TH1: a + b + c + d = 0
=> a + b = - ( c + d ) ; b + c = - ( a + d ) ; c + d = - ( a + b )
Khi đó M = ( -1 ) . 4 = -4
TH2 : a + b + c + d khác 0
=> a = b = c = d
Khi đó M = 1 . 4 = 4
Vậy M = 4 hoặc M = - 4
Tham khảo nhé
https://olm.vn/hoi-dap/tim-kiem?id=1164587&subject=1&q=+++++++++++2a+b+c+da+=a+2b+c+db+=a+b+2c+dc+=a+b+c+2dd+T%C3%ADnh+M=a+bc+d++b+cd+a++c+da+b++d+ab+c+++++++++++
Ta có: \(\frac{2a+b+c+d}{a}=\frac{a+2b+c+d}{b}=\frac{a+b+2c+d}{c}=\frac{a+b+c+2d}{d}\)
\(=\frac{2a+b+c+d+a+2b+c+d+a+b+2c+d+a+b+c+2d}{a+b+c+d}=4\)
=>2a+b+c+d=4a
=>2a=b+c+d
Tương tự ta có:2b=a+c+d 2c=a+b+d 2d=a+b+c
=>2a+2b=b+c+d+a+c+d
=>a+b+2c+2d
=>a+b=2c+2d
\(\Rightarrow\frac{a+b}{c+d}=2\)
Tương tự ta có:\(b+\frac{c}{d}+a=2\)
\(c+\frac{d}{a}+b=2\)
\(d+\frac{a}{b}+c=2\)
=>M=2+2+2+2=8
Bạn tham khảo tại đây:
Câu hỏi của Nguyễn Quỳnh Chi - Toán lớp 7 - Học toán với OnlineMath
\(A=\frac{2a+b+c+d}{a}=\frac{a+2b+c+d}{b}=\frac{a+b+2c+d}{c}=\frac{a+b+c+2d}{d}\)
\(\Rightarrow\frac{a+b+c+d}{a}=\frac{a+b+c+d}{b}=\frac{a+b+c+d}{c}=\frac{a+b+c+d}{d}\)
Với a + b + c + d = 0 => a + b = - ( c + d )
=> \(A=\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)=-4\)
Với \(a+b+c+d\ne0\) => a = b = c = d
=> \(A=1+1+1+1=4\)
Ta có: \(\frac{2a+b+c+d}{a}=\frac{a+2b+c+d}{b}=\frac{a+b+2c}{c}=\frac{a+b+c+2d}{d}\)
\(\Rightarrow\frac{2a+b+c+d}{a}-1=\frac{a+2b+c+d}{b}-1=\frac{a+b+2c+d}{c}-1=\frac{a+b+c+2d}{d}-1\)
\(\Rightarrow\frac{a+b+c+d}{a}=\frac{a+b+c+d}{b}=\frac{a+b+c+d}{c}=\frac{a+b+c+d}{d}\)(1)
TH1: a + b + c + d =0
=> a + b = -c - d
b + c = - a - d
a + c = -b - d
\(\Rightarrow\frac{a+b}{c+d}+\frac{b+c}{a+d}+\frac{c+a}{b+d}\)
\(=\frac{-c-d}{c+d}+\frac{-a-d}{a+d}+\frac{-b-d}{b+d}\)
\(=\frac{-\left(c+d\right)}{c+d}+\frac{-\left(a+d\right)}{a+d}+\frac{-\left(b+d\right)}{b+d}\)
\(=-1+\left(-1\right)+\left(-1\right)=-3\)
TH2: \(a+b+c+d\ne0\)
Từ (1) => a = b = c =d
\(\Rightarrow\frac{a+b}{c+d}+\frac{b+c}{a+d}+\frac{c+a}{b+d}\)
\(=\frac{a+a}{a+a}+\frac{b+b}{b+b}+\frac{c+c}{c+c}\)
\(=1+1+1=3\)
trừ mỗi tỉ lệ cho 1 ta được:
\(\frac{2a+b+c+d}{a}-1=\frac{a+2b+c+d}{b}-1=\frac{a+b+2c+d}{c}-1=\frac{a+b+c+2d}{d}-1\)
\(\Rightarrow\frac{2a+b+c+d}{a}-\frac{a}{a}=\frac{a+2b+c+d}{b}-\frac{b}{b}=\frac{a+b+2c+d}{c}-\frac{c}{c}=\frac{a+b+c+2d}{d}-\frac{d}{d}\)
\(\Rightarrow\frac{a+b+c+d}{a}=\frac{a+b+c+d}{b}=\frac{a+b+c+d}{c}=\frac{a+b+c+d}{d}\)
+Nếu a+b+c+d\(\ne\)0 thì a=b=c=d lúc đó
M=1+1+1+1=4
+Nếu a+b+c+d=0 thì a+b=-(c+d);b+c=-(d+a);c+d=-(a+b);d+a=-(b+c) lúc đó:
M=(-1)+(-1)+(-1)+(-1)=-4
\(\frac{a+b+2c+d}{c}=\frac{a+b+c+2d}{d}=\frac{a+b+2c+d+a+b+c+2d}{c+d}=\frac{2a+2b+3c+3d}{c+d}\)
\(=\frac{2\left(a+b\right)}{c+d}+\frac{3\left(c+d\right)}{c+d}=2.\frac{a+b}{c+d}+3\)
\(\frac{2a+b+c+d}{a}=\frac{a+b+c+2d}{d}=\frac{2a+b+c+d+a+b+c+2d}{a+d}=\frac{3a+3d+2c+2b}{a+d}\)
\(=\frac{3\left(a+d\right)}{a+d}+\frac{2\left(b+c\right)}{a+d}=3+2.\frac{b+c}{a+d}\)
\(\frac{2a+b+c+d}{a}=\frac{a+2b+c+d}{b}=\frac{2a+b+c+d+a+2b+c+d}{a+b}=\frac{3a+3b+2c+2d}{a+b}\)
\(=\frac{3\left(a+b\right)}{a+b}+\frac{2\left(c+d\right)}{a+b}=3+\frac{c+d}{a+b}.2\)
\(\frac{a+2b+c+d}{b}=\frac{a+b+2c+d}{c}=\frac{a+2b+c+d+a+b+2c+d}{b+c}=\frac{3b+3c+2a+2d}{b+c}\)
\(=\frac{3\left(b+c\right)}{b+c}+\frac{2\left(a+d\right)}{b+c}=3+\frac{a+d}{b+c}.2\)
\(\frac{2a+b+c+d}{a}=\frac{a+2b+c+d}{b}=\frac{a+b+2c+d}{c}=\frac{a+b+c+2d}{d}=\frac{5\left(a+b+c+d\right)}{a+b+c+d}=5\)
\(\Rightarrow\frac{2a+b+c+d}{a}+\frac{a+2b+c+d}{b}+\frac{a+b+2c+d}{c}+\frac{a+b+c+2d}{d}=5.4=20\)
\(\Rightarrow3+\frac{a+b}{c+d}.2+3+\frac{b+c}{a+d}.2+3+\frac{c+d}{a+b}.2+3+\frac{d+a}{b+c}.2=20\)
\(\Rightarrow2.\left(\frac{a+b}{c+d}+\frac{b+c}{a+d}+\frac{c+d}{a+b}+\frac{d+a}{b+c}\right)=20-3-3-3-3\)
\(\Rightarrow\frac{a+b}{c+d}+\frac{b+c}{a+d}+\frac{c+d}{b+a}+\frac{d+a}{b+c}=8:2=4\)
vậy \(\frac{a+b}{c+d}+\frac{b+c}{a+d}+\frac{c+d}{a+b}+\frac{d+a}{b+c}=4\)
\(\frac{2a+b+c+d}{a}=\frac{a+2b+c+d}{b}=\frac{a+b+2c+d}{c}=\frac{a+b+c+2d}{d}=\)
\(=\frac{a+b+2c+d+a+b+c+2d}{c+d}=\frac{2\left(a+b\right)}{c+d}+3=\)
Tương tự
\(=\frac{2\left(b+c\right)}{d+a}+3=\)
\(=\frac{2\left(c+d\right)}{a+b}+3=\)
\(=\frac{2\left(d+a\right)}{b+c}+3\)
\(\Rightarrow\frac{2\left(a+b\right)}{c+d}+3=\frac{2\left(b+c\right)}{d+a}+3=\frac{2\left(c+d\right)}{a+b}+3=\frac{2\left(d+a\right)}{b+c}+3\)
\(\Rightarrow\frac{2\left(a+b\right)}{c+d}=\frac{2\left(b+c\right)}{d+a}=\frac{2\left(c+d\right)}{a+b}=\frac{2\left(d+a\right)}{b+c}=\)
\(=\frac{2\left(a+b\right)+2\left(b+c\right)+2\left(c+d\right)+2\left(d+a\right)}{c+d+d+a+a+b+b+c}=\frac{4\left(a+b+c+d\right)}{2\left(a+b+c+d\right)}=2\)
\(\Rightarrow\frac{a+b}{c+d}+\frac{b+c}{d+a}+\frac{c+d}{a+b}+\frac{d+a}{b+c}=1+1+1+1=4\)