必要性：f(x1,...,xn)=（a1x1+...+anxn)(b1x1+...+bnxn),若向量a＝(a1 a2 ...an)^T和b＝（b1 b2 ...bn）^T线性无关,则可将其扩充为R^n的一组基,再做变量替换y1=a1x1+...+anxn,y2=b1x1+...+bnxn,y3,...,yn由基中其余...