令t=x^2+2x 原式变为：t^2-3t-4=0 (t-4)(t+1)=0 t=4或t=-1 x^2+2x=4或x^2+2x=-1 (x+1)^2=5或（x+1）^2=0 x=根号5-1,或-根号5-1,或-1