Let V {\displaystyle {}V} be a finite-dimensional K {\displaystyle {}K} -vector space over a field K {\displaystyle {}K} , and let L , L 1 , … , L m {\displaystyle {}L,L_{1},\ldots ,L_{m}} denote linear forms on V {\displaystyle {}V} . Show that the relation
holds if and only if L {\displaystyle {}L} belongs to the linear subspace (in the dual space) generated by the L 1 , … , L m {\displaystyle {}L_{1},\ldots ,L_{m}} .