a, Ta có : M-1= \(\frac{a^2+b^2-c^2}{2ab}-1+\frac{b^2+c^2-a^2}{2bc}-1+\frac{a^2+c^2-b^2}{2ac}+1\)=\(\frac{\left(a-b\right)^2-c^2}{2ab}+\frac{\left(b-c\right)^2-a^2}{2bc}+\frac{\left(a+c\right)^2-b^2}{2ac}\)
=\(\frac{\left(a-b-c\right)\left(a-b+c\right)}{2ab}+\frac{\left(b-c-a\right)\left(b-c+a\right)}{2bc}+\frac{\left(a+c-b\right)\left(a+b+c\right)}{2ac}\)
=\(\frac{\left(a-b-c\right)\left(a-b+c\right)c+\left(b-c-a\right)\left(b-c+a\right)a+\left(a+c-b\right)\left(a+c+b\right)b}{2abc}\)
=\(\frac{\left(ac-bc-c^2\right)\left(a-b+c\right)-\left(a+c-b\right)\left(ba-ca+a^2\right)+\left(a+c-b\right)\left(ab+bc+b^2\right)}{^{ }2abc}\)
=\(\frac{\left(a+c-b\right)\left(ac-bc-c^2-ba+ca-a^2+ab+bc+b^2\right)}{^{ }2abc}\)
=\(\frac{\left(a+c-b\right)\left[b^2-\left(a-c\right)^2\right]}{2abc}=\frac{\left(a+c-b\right)\left(b-a+c\right)\left(b+a-c\right)}{2abc}\) (*)
a, vì a,b,c là độ dài 3 cạnh của 1 tam giác nên a,b,c>0 và a+b-c>,a+c-b>0,
b+c-a>0 \(\Rightarrow\) (*) >0 nên M-1>0 \(\Rightarrow\)M>0
b,Với M=1, ta có M-1 = (*)=0 \(\Rightarrow\)(a+c-b)(b-a+c)(b+a-c)=0
\(\Leftrightarrow\left[\begin{matrix}a+b=c\\a+c=b\\b+c=a\end{matrix}\right.\)
. TH1 : a+b=c\(\Rightarrow\) \(\frac{a^2+b^2-c^2}{2ab}-1=\frac{\left(a-b\right)^2-\left(a+b\right)^2}{2ab}=\frac{-4ab}{2ab}=-2\)\(\Rightarrow\frac{a^2+b^2-c^2}{2ab}=-1\)
mặt khác a+b=c thì a-c=b \(\Rightarrow\frac{a^2+c^2-b^2}{2ac}+1=\frac{\left(a+c\right)^2-\left(a-c\right)^2}{2ac}=\frac{4ac}{2ac}=2\)
\(\Rightarrow\frac{a^2+c^2-b^2}{2ac}=1\)\(\Rightarrow\frac{b^2+c^2-a^2}{2bc}=1\)(đpcm)
. TH2 và TH3 tương tự như trường hợp 1 ta chứng minh được bài toán