let V be RealUnitarySpace; :: thesis: for W1, W2, W3 being Subspace of V st W1 is Subspace of W3 & W2 is Subspace of W3 holds
W1 + W2 is Subspace of W3
let W1, W2, W3 be Subspace of V; :: thesis: ( W1 is Subspace of W3 & W2 is Subspace of W3 implies W1 + W2 is Subspace of W3 )
assume A1: ( W1 is Subspace of W3 & W2 is Subspace of W3 ) ; :: thesis: W1 + W2 is Subspace of W3

now :: thesis: for v being VECTOR of V st v in W1 + W2 holds
v in W3

let v be VECTOR of V; :: thesis: ( v in W1 + W2 implies v in W3 )
assume v in W1 + W2 ; :: thesis: v in W3
then consider v1, v2 being VECTOR of V such that
A2: ( v1 in W1 & v2 in W2 ) and
A3: v = v1 + v2 by RUSUB_2:1;
( v1 in W3 & v2 in W3 ) by A1, A2, RUSUB_1:1;
hence v in W3 by A3, RUSUB_1:14; :: thesis: verum

end;

hence W1 + W2 is Subspace of W3 by RUSUB_1:23; :: thesis: verum