F.3 Maths!!Extremely Urgent!15 points~

1) 2^(n+3)-2^(n+1)/ 2^(n+2)
= {2^(n+1)[4-1]/2^(n+2)(2)}
=[4-1]/2=3/2

2)3^(2n-1)X9^(n-2)/ [6^(n+1)]^2
= 3^(2n-1)x 3^2(n-2)/6^(2n+2)
=3^(2n-1+2n-4)/2^(2n+2)．3^(2n+2)
=3^(4n-5-2n-2)/2^(2n+2)
=3^(2n-7)/2^(2n+2)

3){ 3/x + 2/y=14
{ 5/x - 3/y =17
take 1/x=a, 1/y=b, then we have
3a+2b=14----(1)
5a-3b=17-----(2)
3x(1)+2x(b), we have
9a+6b+10a-6b=42+34
19a=76
a=4 put it into (1), we have
3(4)+2b=14
2b=2
b=1
therefore
x=1/a=1/4
y=1/b=1



2008-02-13 16:48:45 補充：
對唔住，有少少打錯1) 2^(n 3)-2^(n 1)/ 2^(n 2) 分子、分母各抽2^(n 1)；然後相約= {2^(n 1)[4-1]/2^(n 1)(2)}（相約後，分子餘下（４－１）；分母餘下２）=[4-1]/2=3/2

2008-02-13 16:50:55 補充：
yahoo食了＋號1) 2^(n＋3)-2^(n＋1)/ 2^(n＋2) 分子、分母各抽2^(n＋1)；然後相約= {2^(n＋1)[4-1]/2^(n＋1)(2)}（相約後，分子餘下（４－１）；分母餘下２）=[4-1]/2=3/2

1)2^(n+3)-2^(n+1)/ 2^(n+2)
=2^n(2^3)-2^n(2)/2^n(2^2)
=2^n(8-2)/2^n(2^2)
=6/4
=3/2
2)3^(2n-1)X9^(n-2)/ [6^(n+1)]^2
=3^(2n-1)x3^2n-4/ 6^(2n+2)
=3^2n+2n-1-4/6^(2n+2)
=3^4n-5/ 6^(2n+2)
=3^4n-5/2^(2n+2)x3^(2n+2)
=3^4n-2n-5+2/2^(2n+2)
=3^(2n-7)/ 2^(2n+2)
3)唔識