如果圆半径为r，圆弧对应的圆心角为@,则有r×r=0.75×0.75+(r-0.45)×(r-0.45)r=0.85sin(@/2)=0.75/0.85=15/17@=2arcsin(15/17)弧长：l=r×@面积：s=r×r×@×1/2
