more about factorization[help]

x^2 - y^2 + 4x + 8y -12
= x^2 + 4x - (y^2 - 8y + 12)
= x^2 + 4x - (y - 2)(y - 6)
= x^2 + [(y - 2) - (y - 6)]x - (y - 2)( y - 6)
= [x - (y - 6)][x + (y - 2)]
=(x - y + 6)(x + y -2)
No need to memorise the tax rate and allowance.

You do not need to memorise the tax rate and allowance as it will varies.
But it is VERY IMPORTANT to know the method to calculate the salary tax because it may appears in your exam paper(even open exam)!!!

2009-07-22 16:25:02 補充：
Interest rate(simple and compound interest) problem(it should be in your text book) may also appears in your exam paper!!!

You should memorise the method to calculate the salary tax and interest.

Many people forget this after they finish form 3.

x^2-y^2+4x+8y-12
=x^2+4x+4-(y^2-8y+16)
=(x+2)^2-(y-4)^2
=(x+2+y-4)(x+2-y+4)
=(x+y-2)(x-y+6)

x^2 - y^2 + 4x + 8y - 12
= x^2 + 4x – 5 – y^2 + 8y – 7
= x^2 + 4x – 5 – (y^2 – 8y + 7)
= (x – 1)(x + 5) – (y –1)(y-7)

if you are studying maths only
no need to memorize the 'salary tax allowance' or 'salaries tax rate'.