26.
so we are given A is subset of C and B is subset of D and we have to prove A$A\times B is subset of C\times D$
now to prove this let us assume we have a pair (x,y), where x belongs to A and y belongs to B
so we have to established that (x,y) belongs to A\times B so to establish this let us take an ordered pair (x,y) such that we can write it as x belongs to A and y belongs to B . now as x\epsilon A therefore it will belong to C as A is subset of C
and same goes for y
$so (x,y) \varepsilon C\times D$
as (x,y) is any ordered pair which belong to $A\varepsilon B$ and we have proved that it also belongs to $C\varepsilon D$
for more clarlity.. watch trev tutor videos on youtube for direct proof
