you make the them the same
remember that x/x=1 when x=x
so
make the bottoms the same
find the least common multipule
they don't have any common factors so just multiply them
ab is the common denomenator
so multiply 1/a by b/b to get b/(ab)
multiply 1/b by a/a to get a/(ab)
now we have
b/(ab)+a/(ab)=(b+a)/(ab) or (a+b)/(ab)
