let p, q, r, s be ExtReal; :: thesis: ( p <= r & s <= q implies [.r,s.[ c= [.p,q.] )
A1: [.r,s.[ c= [.r,s.] by Th24;
assume that
A2: p <= r and
A3: s <= q ; :: thesis: [.r,s.[ c= [.p,q.]
[.r,s.] c= [.p,q.] by A2, A3, Th34;
hence [.r,s.[ c= [.p,q.] by A1; :: thesis: verum