怎么用javascipt写一个钢卷外径公式，π*（D*D-d*d）*b*ρ/4= D=根号下（4G/（π*b*ρ）+d*d）

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Q1：javascipt截取value的第一个字符怎么样写？var value=$.trim($(“#Remark44”).val())

value.substring(0,1)

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