当x≠ ±4,分式1/x平方-16有意义 1/r1-1/r2 =(r2-r1)/r1*r2 m+3/m平方-9-3/3-m =(m+3)/(m+3)(m-3)+3/(m-3) =1/(m+3)+3/(m+3) =4/(m+3)