help with specific trinomials.

right, so i know how to do trinomials in a normal case with only one pronumeral.

a^2 + 2a + 1

(a+1)(a+1)

and i can do non-monic trinomials no problem too, problem for me is when i discover a trinomial which has 2 pronumerals.

a^2 + 2ab + b^2

now i have no idea how i'm meant to work it out.

i know that this simplifies to (a+b)(a+b) because i can do the expansion for it.

but i have no idea how to simplify it and if it were slightly more complex

eg. k^2 + 8kp + p^2 i would not know what to do in the exam.

help please! could someone please show me how to work out these expressions a^2 + 2ab + b^2

thankyou very much in advance. :)