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求圆弧长度计算公式,已知切面跨度长800CM,切面高280CM.求圆弧长.请...

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求圆弧长度计算公式,已知切面跨度长800CM,切面高280CM.求圆弧长.请...
设圆半径为r弦长为d弦高为h所对圆心角为t,则依相交弦定理得(d/2)^2=(2r-h)h ==> h=d^2/(8h)+h/2,故t=2arcsin{(d/2)/[d^2/(8h)+h/2]}.因此,弧长L=rt=[d^2/(8h)+h/2]*2arcsin{(d/2)/[d^2/(8h)+h/2]}.
d=800,h=280,
r²=d²/4+(r-h)²
r²=160000+r²-560r+280*280
560r=238400
r=2980/7
L=[d^2/(8h)+h/2]*2arcsin{(d/2)/[d^2/(8h)+h/2]}
=(640000/(8*280)+140)*2arcsin{(800/2)/[800^2/(8*280)+280/2]}
=(640000/(8*280)+140)*2*1.22145192877842
=1039.97907078848cm