let K be Field; :: thesis: for V being VectSp of K
for W1, W2 being Subspace of V
for I1 being Basis of W1
for I2 being Basis of W2 st W1 /\ W2 = (0). V holds
I1 \/ I2 is Basis of (W1 + W2)

let V be VectSp of K; :: thesis: for W1, W2 being Subspace of V
for I1 being Basis of W1
for I2 being Basis of W2 st W1 /\ W2 = (0). V holds
I1 \/ I2 is Basis of (W1 + W2)

let W1, W2 be Subspace of V; :: thesis: for I1 being Basis of W1
for I2 being Basis of W2 st W1 /\ W2 = (0). V holds
I1 \/ I2 is Basis of (W1 + W2)

let I1 be Basis of W1; :: thesis: for I2 being Basis of W2 st W1 /\ W2 = (0). V holds
I1 \/ I2 is Basis of (W1 + W2)

let I2 be Basis of W2; :: thesis: ( W1 /\ W2 = (0). V implies I1 \/ I2 is Basis of (W1 + W2) )
assume P1: W1 /\ W2 = (0). V ; :: thesis: I1 \/ I2 is Basis of (W1 + W2)
set I = I1 \/ I2;
reconsider W = W1 + W2 as strict Subspace of V ;
reconsider WW1 = W1, WW2 = W2 as Subspace of W by VECTSP_5:7;
( the carrier of WW1 c= the carrier of W & the carrier of WW2 c= the carrier of W ) by VECTSP_4:def 2;
then ( I1 c= the carrier of W & I2 c= the carrier of W ) ;
then reconsider I0 = I1 \/ I2 as Subset of W by XBOOLE_1:8;
reconsider I10 = I1 as Basis of WW1 ;
reconsider I20 = I2 as Basis of WW2 ;
A2: WW1 /\ WW2 is Subspace of V by VECTSP_4:26;
A3: WW1 + WW2 is Subspace of V by VECTSP_4:26;
for x being object holds
( x in WW1 /\ WW2 iff x in (0). V )
proof
let x be object ; :: thesis: ( x in WW1 /\ WW2 iff x in (0). V )
hereby :: thesis: ( x in (0). V implies x in WW1 /\ WW2 )
assume x in WW1 /\ WW2 ; :: thesis: x in (0). V
then ( x in WW1 & x in WW2 ) by VECTSP_5:3;
hence x in (0). V by ; :: thesis: verum
end;
assume x in (0). V ; :: thesis: x in WW1 /\ WW2
then ( x in W1 & x in W2 ) by ;
hence x in WW1 /\ WW2 by VECTSP_5:3; :: thesis: verum
end;
then for x being Vector of V holds
( x in WW1 /\ WW2 iff x in (0). V ) ;
then A4: WW1 /\ WW2 = (0). V by
.= (0). W by VECTSP_4:36 ;
for x being object holds
( x in W iff x in WW1 + WW2 )
proof
let x be object ; :: thesis: ( x in W iff x in WW1 + WW2 )
hereby :: thesis: ( x in WW1 + WW2 implies x in W )
assume x in W ; :: thesis: x in WW1 + WW2
then consider x1, x2 being Vector of V such that
B2: ( x1 in W1 & x2 in W2 & x = x1 + x2 ) by VECTSP_5:1;
x1 in W1 + W2 by ;
then reconsider xx1 = x1 as Vector of W ;
x2 in W1 + W2 by ;
then reconsider xx2 = x2 as Vector of W ;
x = xx1 + xx2 by ;
hence x in WW1 + WW2 by ; :: thesis: verum
end;
assume x in WW1 + WW2 ; :: thesis: x in W
then consider x1, x2 being Vector of W such that
B2: ( x1 in WW1 & x2 in WW2 & x = x1 + x2 ) by VECTSP_5:1;
thus x in W by B2; :: thesis: verum
end;
then for x being Vector of V holds
( x in W iff x in WW1 + WW2 ) ;
then W = WW1 + WW2 by ;
then I0 is base by ;
hence I1 \/ I2 is Basis of (W1 + W2) ; :: thesis: verum