certain condition you might lose marks (like if you are asked to show
a result that you could work backwards from to something that you jump
to with little explanation).
(and if you can do that jump in your head, you don't need our advice)
i only want to know if there are some 'way' to do it quickly without writing the full expansion, or some way for me to do the mental calculation. for example, by the identity a^3 + b^3 = (a + b) (a^2 - ab + b^2), i do not need to do all the expansion when i meet a similar pattern in a question. i am afraid that i won't have enough time in the exam. but thanks for your advice anyway