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\(\frac{b+c+d}{\left(b-a\right)\left(c-a\right)\left(d-a\right)\left(x-a\right)}=\frac{\left(a+b+c+d-x\right)+\left(x-a\right)}{\left(b-a\right)\left(c-a\right)\left(d-a\right)\left(x-a\right)}\)\(=\frac{\left(a+b+c+d-x\right)}{\left(b-a\right)\left(c-a\right)\left(d-a\right)\left(x-a\right)}+\frac{1}{\left(b-a\right)\left(c-a\right)\left(d-a\right)}\)
Áp dụng hoán vị vòng \(b\rightarrow c\rightarrow d\rightarrow a\rightarrow b\) vào VT , ta được :
\(\left(a+b+c+d-x\right)\)[\(\frac{1}{\left(a-b\right)\left(a-c\right)\left(a-d\right)\left(a-x\right)}+\frac{1}{\left(b-a\right)\left(b-c\right)\left(b-d\right)\left(b-x\right)}+\frac{1}{\left(c-a\right)\left(c-b\right)\left(c-d\right)\left(c-x\right)}\)\(+\frac{1}{\left(d-a\right)\left(d-b\right)\left(d-c\right)\left(d-x\right)}\).
Quy đồng mẫu thức và tính toán biểu thức trong [ ] ta được :
\(\frac{-1}{\left(x-a\right)\left(x-b\right)\left(x-c\right)\left(x-d\right)}\)
Vậy ...............
\(\left(a+b+c+d\right)\left(a-b-c+d\right)=\left(a-b+c-d\right)\left(a+b-c-d\right)\)
\(\Rightarrow\left[\left(a+d\right)+\left(b+c\right)\right]\left[\left(a+d\right)-\left(b+c\right)\right]-\left[\left(a-d\right)-\left(b-c\right)\right]\left[\left(a-d\right)+\left(b-c\right)\right]=0\)
\(\Rightarrow\left(a+d\right)^2-\left(b+c\right)^2-\left(a-d\right)^2+\left(b-c\right)^2=0\)
\(\Rightarrow a^2+d^2+2ad-b^2-c^2-2bc-a^2-d^2+2ad+b^2+c^2-2bc\)
\(\Rightarrow4ad-4bc\)
\(\Rightarrow ad=bc\Rightarrow\frac{a}{c}=\frac{b}{d}\)
Ta có:
\(\left(a+b\right)\left(b+c\right)\left(c+d\right)\left(d+a\right)\)
\(=\left(\frac{2017}{c}+\frac{2017}{d}\right)\left(\frac{2017}{d}+c\right)\left(c+d\right)\left(d+\frac{2017}{c}\right)\)
\(=\frac{2017}{c^2d^2}\left(c+d\right)^2\left(cd+2017\right)^2\)
\(=\frac{2017}{c^2d^2}\left(c^2d+d^2c+2017c+2017d\right)^2\left(1\right)\)
Ta lại có:
\(\left(a+b+c+d\right)^2\)
\(=\left(\frac{2017}{c}+\frac{2017}{d}+c+d\right)^2\)
\(=\frac{1}{c^2d^2}\left(c^2d+d^2c+2017c+2017d\right)^2\left(2\right)\)
Từ (1) và (2) \(\Rightarrow M=2017\)
