初中數學,唔識做... (1)

1. a=(b+ce)/(de-f) [e]
a(de-f)=b+ce
ade-af=b+ce
ade-ce=af+b
e(ad-c)=af+b
e=(af+b)/(ad-c)//

2. (a+b)²+(a-b)²
=(a²+2ab+b²)+(a²-2ab+b²)
=a²+2ab+b²+a²-2ab+b²
=a²+a²+2ab-2ab+b²+b²
=2a²+2b²
=2(a²+b²) //
注:要記住兩條identites: (a+b)²=a²+2ab+b², (a-b)²=a²-2ab+b²

3. AD= 3-(-2)= 5, BC= 1-(-2)= 3, CD= 3-(-1)= 4
AB²=4²+2² (pyth. theorem)
AB² =16+4
AB=√(20)
AB=2√5

perimeter=AD+BC+CD+AB
=5+3+4+2√5
=12+2√5
=16.47// (corr. to 2 d.p.)

4. 同第三題一樣?
5. (a²-2a)/(b-3a), 你想計乜?