OP → = OQ → + QR → + RS → + SP → = OQ → + QR → + RS → + 1 2 SQ → =−c−b−a+ 1 2 c+a =− 1 2 a−b− 1 2 c
OT → =−b より
OP → = OT → + TP → =−b− 1 2 c+a =− 1 2 a−b− 1 2 c