Maths唔識抵死題(new)

Q1
first term a=22, common difference d=-4,
sum of first n terms=n/2 [2a+(n-1)d]=64
n/2 (2(22)+(n-1)(-4))=64
n/2 (48-4n)=64
24n-2n^2=64
n^2-12n+32=0
(n-4)(n-8)=0
n=4 or 8

Q2
Let the total output last year be $ x million.
The total cost of last year=$ x-6 million
The total output last year be $ 1.3x million.
The total cost of last year=$ 0.8(x-6) million.
0.8(x-6)+10=1.3x
5.2=0.5x
x=10.4
the total output last year is $ 10.4 million.

Q3
a,b,c,d form a geometric sequence.
Let b=ar,c=ar^2,d=ar^3.
a-c=8
b-d=72

a-ar^2=8...........(1)
ar-ar^3=72........(2)

(2)/(1):
r=9
So b/a=r=9

2010-12-29 20:21:02 補充：
Additional information to each questions:

Q1
common difference d =14-18= -4

Q2
The total output this year be $ 1.3x million.
The total cost of this year=$ 0.8(x-6) million.
Total output this year is 10 million more than total cost this year,
0.8(x-6)+10=1.3x
x=10.4

2010-12-29 20:21:09 補充：
Q3
a,b,c,d form a geometric sequence.
Let b=ar,c=ar^2,d=ar^3, r is the common difference.
b=ar => b/a=r=9.

Please explain each of them briefly!