在平面几何中,有一类结论为形如“1/a+1/b=2/c”的命题,这类命题的证明难度较大,证法灵活,似无章可循.本文以一道高等几何背景题为例分别利用共边定理和张角公式给出两种初等证法,这些方法对于证明结论为形如“1/a+1/b=2/c”的命题有一定的借鉴作用. In plane geometry, there are some kinds of conclusions that are propositions such as “1 / a + 1 / b = 2 / c ”, which prove more difficult and more flexible. An example of a higher geometric background, for example, gives two kinds of elementary proofs by using the common-edged theorems and the open-angle formulas respectively. These methods are valid for propositions proving that the conclusion is like “1 / a + 1 / b = 2 / c” A certain reference.