bài 2:

\(S=\dfrac{1}{1+x1+x1x2}+\dfrac{1}{1+x2+x2x3}+\dfrac{1}{1+x3+x3x1}\)

=\(S=\dfrac{1}{1+x1+x1x2}+\dfrac{x1}{x1\left(1+x2+x2x3\right)}+\dfrac{x1x2}{x2x1\left(1+x3+x3x1\right)}\)

S=\(\dfrac{1}{x+x1+x1x2}+\dfrac{x1}{x1+x1x2+1}+\dfrac{x1x2}{x1x2+1+x1}\)

S=\(\dfrac{1+x1+x1x2}{x1x2+1+x1}=1\)

chúc bạn học tốt ^^