令弦长为l，半径r=280/2=140l^2=r^2+r^2-2*r*r*cos(360°/8) =2r^2-2r^2cos45° =(2-√2)*140^2 l=140*√(2-√2)≈107.15
半径140中心角 360÷8＝4545÷2＝22.5弦长 2x140xsin22.5=280x0.3827=107.16
