數學唔識,唔該幫幫忙,十萬火急呀!!

1. (x+y)²-(x-y)²
= [(x+y)(x+y)]-[(x-y)(x-y)]
= (x²+2xy+y²) - (x²-2xy+y²)
= x²+2xy+y²-x²+2xy-y²
= x²-x²+2xy+2xy+y²-y²
= 4xy

2. 蘋果3個: $3m; 橙4個: $4n
應找回: $(20-3m-4n)

1. (x+y)²-(x-y)² = x² + 2xy +y² - x² + 2xy - y² = 4xy

2. 20 - 3m -4n