10长直导线弯成如图所示形状,中间部分半径为R,圆心角为 dfrac (2)(3)pi ,则圆弧中心点O处的磁-|||-感应强度大小为 () 。-|||-6-|||-旦-|||-dfrac ({mu )_(0)I}(pi R)(1-dfrac (sqrt {3)}(2)+dfrac (pi )(6))-|||-dfrac ({mu )_(0)I}(pi R)(dfrac (1)(2)+dfrac (pi )(6))-|||-dfrac ({mu )_(0)I}(pi R)(1-dfrac (sqrt {3)}(2)+dfrac (2pi )(3))-|||-dfrac ({mu )_(0)I}(pi R)(sqrt (3)+dfrac (pi )(6))