let R be Ring; :: thesis: for V being RightMod of R
for L1, L2, L3 being Linear_Combination of V holds L1 + (L2 + L3) = (L1 + L2) + L3
let V be RightMod of R; :: thesis: for L1, L2, L3 being Linear_Combination of V holds L1 + (L2 + L3) = (L1 + L2) + L3
let L1, L2, L3 be Linear_Combination of V; :: thesis: L1 + (L2 + L3) = (L1 + L2) + L3
let v be Vector of V; :: according to RMOD_4:def 8 :: thesis: (L1 + (L2 + L3)) . v = ((L1 + L2) + L3) . v
thus (L1 + (L2 + L3)) . v = (L1 . v) + ((L2 + L3) . v) by Def9
.= (L1 . v) + ((L2 . v) + (L3 . v)) by Def9
.= ((L1 . v) + (L2 . v)) + (L3 . v) by RLVECT_1:def 3
.= ((L1 + L2) . v) + (L3 . v) by Def9
.= ((L1 + L2) + L3) . v by Def9 ; :: thesis: verum