众所周知:如果一个数列的各项倒数成等差数列,则此数列叫做调和数列。下面介绍应用张角公式来证明有关线段 a、b、c 成调和数列的问题,这种解法对于解这类题具有普遍意义。一、张角公式如图,设直线 ACB 外一视点 P 对于线段 AC、CB 的张角分别为α、β,且α+β<180°, It is well known that if the inverse of each series is an arithmetic progression, the sequence is called a harmonic sequence. The following is a description of the application of the Zhang Jiao formula to prove the problem of the line segments a, b, and c into harmonic sequences. This solution has universal significance for solving such problems. First, the formula for the angle of the figure is shown in the figure. Let the line of sight outside the line ACB be P. For the line of the line, the opening angles of the AC and CB are α and β, respectively, and α+β<180°.