[this wednesday 交]form two maths .. plz help me ..

form two maths ... i dunno how to do it ..
so .... plz help me ..
i need to hand in this friday ... thz =]


new one :
A surveyor wants to measure the width of a river. He mariks two points A and B on opposite sides of the river and then a point C 20m from point B along the riverbank. If < ABC = 90° and <BCA = 68°12' , how wide is the river ?

[[i] 本帖最後由 kate`;} 於 2007-8-20 04:46 PM 編輯 [/i]]

1)
48/(v-x)=4
48/(v+x)=3

12 = v-x
36 = 3v-3x
48 = 3v+3x
72 = 6v
v = 12
x = 3

2)
5/m +6/n = 1
4/3m - 2/n = 8/3

4/m - 6/n = 8
5/m + 6/n =1
9/m=9
m=1
5+6/n = 1
n = -3/2

3)
let x be the age of father now

x/3 + 1 + 5 = (x+5)/2 - 4
x/3 + 10 = (x+5)/2
2x/3 + 20 = x +5
15 = x/3
x = 45

4)
x+3y=5
6x+16y=27

6x + 18y = 30
6x + 16y = 27
2y = 3
y = 3/2
x = 1/2

A surveyor wants to measure the width of a river. He mariks two points A and B on opposite sides of the river and then a point C 20m from point B along the riverbank. If < ABC = 90° and <BCA = 68°12' , how wide is the river ?
the width of a river = AB
BC = 20m
AB/BC = tan 68°12'
AB = 34.92m