On 97.14: Hiroshi Okumura writes: The last sentence beginning with "If the four points P, A, Q, B" is incomplete, also Figure 5 is wrong. They should be as follows: If the four points P, A, Q, B lie in this order, the left-hand side of (1) equals the sum of the area surrounded by (AP). (PQ) and the segment AQ and the area surrounded by (AB), (BQ) and AQ, which equals the hatched area in Figure 5.