lgx平方-lg(x+2)=0 ==> lg [x^2 / (x+2)] = 0 ==> x^2 / (x +2) = 1 ==> x^2 = x +2 ==> x^2 - x -2 = 0 ==> (x -2)(x +1) = 0 ==> x + 1 = 0 ==> x =2,-1