MATHS EASY,f.2(10)!!!

Proof :
(-3r+2s)(3r+2s)
=(2s+3r)(2s-3r)
=4s^2-9r^2

let a = -2s and b = -3r
(-3r+2s)=-(-2s+3r)
(-3r+2s)(3r+2s)
=-(-2s+3r)(3r+2s)
=-(9r^2-4s^2)
=4s^2-9r^2

Also
(2x-1)(y+2x-1)=2xy-y-(2x-1)(1-2x)
=2xy-y-(2x-4x^2-1+2x)
=2xy-y-4x+4x^2+1
=4x^2-4x-2xy+1

阿 Sir, 你真的明她在問甚麼嗎？
我跟 Kwok Sir 一樣，一點也不明！

你是否真的明白她問什麽？

我一D都睇唔明？

(-3r+2s)(3r+2s) 也可以用

(-3r+2s)(3r+2s)
= -(3r-2s)(3r+2s)
= -[(3r)² - (2s)²]
= -(9r² - 4s²)
= -9r² + 4s²