联立方程组中分子的维数是不是一定要相等，比如下面的方程中分子有一元，有三元的就求不出来：

STEM.formula<-STEM~ a1*DBH^b1/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3*H^c3*CW^d3 + a4*DBH^b4*H^c4*CW^d4)*( a5*DBH^b5)

SKIN.formula<-SKIN~ a2*DBH^b2/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3*H^c3*CW^d3 + a4*DBH^b4*H^c4*CW^d4)*( a5*DBH^b5)

LEAF.formula<-LEAF~ a3*DBH^b3*H^c3*CW^d3/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3*H^c3*CW^d3 + a4*DBH^b4*H^c4*CW^d4)*( a5*DBH^b5)

BRANCH.formula<-BRANCH~ a4*DBH^b4*H^c4*CW^d4/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3*H^c3*CW^d3 + a4*DBH^b4*H^c4*CW^d4)*( a5*DBH^b5)

ABOVE.formula<- ABOVE~ a5*DBH^b5/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3*H^c3*CW^d3 + a4*DBH^b4*H^c4*CW^d4)*( a5*DBH^b5)

必须改成这种形式

STEM.formula<-STEM~ a1*DBH^b1/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3+ a4*DBH^b4)*( a5*DBH^b5)

SKIN.formula<-SKIN~ a2*DBH^b2/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3+ a4*DBH^b4)* (a5*DBH^b5)

LEAF.formula<-LEAF~ a3*DBH^b3/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3+ a4*DBH^b4)*( a5*DBH^b5)

BRANCH.formula<-BRANCH~ a4*DBH^b4/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3+ a4*DBH^b4)*(a5*DBH^b5)

ABOVE.formula<- ABOVE~ a5*DBH^b5/( a1*DBH^b1+ a2*DBH^b2+

             a3*DBH^b3+ a4*DBH^b4)* (a5*DBH^b5)