為何直線互相垂直的時侯,斜率相乘是 -1???

I think the solution should involve some A.MATHS knowledge.
From the formula , tanθ=│(m1-m2)/(1+m1m2)│
For two straight lines to be perpendicular each other , then θ=90
i.e. 1+m1m2=0
This results in m1m2=-1

2009-09-04 00:15:33 補充：
θ is the angle between two straight lines
m1 and m2 are the slope of two straight lines respectively

Using Pythagoras''s theorem is simple and straight forward.

Consider a right-angle Isosceles triangle which the right-angle vertex is at original (with out lost of generality).

2009-09-12 12:49:12 補充：
Let's (x1,y1), (x2, y2) and (0,0) are the vertex points.

x1^2+y^2 = x2^2+y2^2
2(x1^2+y1^2) = (x1-x2)^2 + (y1-y2)^2

2009-09-12 12:51:30 補充：
其實它們是對方的opposite所以才互相垂直

<<<--- no such thing in mathematics, rather like imagination. i am afraid to say it is not a good way to learn mathematics.

其實它們是對方的opposite所以才互相垂直
e.g. a-b opposite is b-a
because a-b*-1=-a--b=-a+b=b-a

for any linear equation with straight line,
#Ax+By=c perpendicular to
@Bx-Ay=c where c can be any number


#(slope=-A/B)
@(slope=-B/-A=B/A)



eg: pretty*-1=pretty*1+negative sign
pretty*1=pretty
-pretty=ugly