Equations of Straight Lines

1. 45度對面個隻角都係45° (vert. opp. ∠s)
Then,
tan 45° = y/-2
1=y/-2
y=-2

∴The y-intercept of L is (0,-2)

By considering the general formula of straight line,
L: y=mx+c
The slope of L=m
m=[(-2)-0]/[0-(-2)]
m=-1
Therefore, the equation of L:
y=-x-2
x+y+2=0

So, A is the answer.

2.
tan 45°=y/4
1=y/4
y=4

∴The y-intercept of L is (0,4)

By considering the general formula of straight line,
L: y=mx+c
The slope of L=m
m=(4-0)/(0-4)
m=-1
Therefore, the equation of L:
y=-x+4
x+y=4

So, A is the answer.

2014-12-17 00:22:08 補充：
3.

∵P lies on x=y & AP=PB.
By distance formula,
L.H.S.=√[(-2-2)^2 +(-2-5)^2]
=√65

R.H.S.=√[(-2-6)^2 + (-2+3)^2]
=v65

∵L.H.S.=R.H.S.
∴The coordinate of P is (-2,-2)
The answer is A.

40哥題要用番題目佢比哥d資料
x=y 同埋 AP=PB

2014-12-17 00:38:55 補充：
Other method：

https://s.yimg.com/rk/HA07344914/o/130790260.png