= \({1 \over {{x^{b + c}} + {x^{c + a}} + {x^{a + b}}}}\sum\limits_{}^{} {{x^{b + c}}} \)
= \({1 \over {{x^{b + c}} + {x^{c + a}} + {x^{a + b}}}}\,({x^{b + c}} + {x^{c + a}} + {x^{a + b}}) = 1\).