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I know how to construct U1, U2 and U1 intersect U2, but how do i construct U1 + U2?

U1 is the matrix of L1 and I just row reduced it etc.

U2 is the matrix of L2 and i just row reduced it etc.

U1 intersect U2 is the matrices added together (making it 4 rows), and row reducing it.

But how do i do U1+U2? Thanks.