Hi Nrt,

The first equation

is an identity-- it is true for all values of a and b.

Now

so

.

Now let and , so .

Then the identity becomes

so

.

This is the result you were asked to show, except that I used x and y instead of a and b. It seems less confusing to me that way. (I get confused very easily). But you can go back and replace x and y with a and b if you prefer it that way.