sin∠PAQ＝-cos(∠PAB+∠DAQ)＝-cos∠PABcos∠DAQ+sin∠PABsin∠DAQ＝(1/AP)(1/AQ)+(BP/AP)(DQ/AQ)＝(1+BP×DQ)/(AP×AQ)＝(2/2+BP×DQ)/(AP×AQ)＝[(AP^2-BP^2+AQ^2-DQ^2)/2+BP×DQ]/(AP×AQ)＝(AP^2+AQ^2-PQ^2)/(2...