2."无限长"导线弯成如图所示的两种形状,通过的电-|||-流为I,若分别用B和Bb表示两种形状载流导线在O点-|||-产生的磁感应强度的大小,则 _(a)= __ dfrac ({mu )_(0)l}(6R)+dfrac ({mu )_(0)l}({pi )^2R}(1-dfrac (sqrt {3)}(2))-|||-____-|||-_(b)=dfrac ({mu )_(0)l}(4R)+dfrac ({mu )_(0)l}(4pi R) o-|||-I I-|||-I 120°-|||-I B-|||-分析: 6-- II s-|||-,