doubt in perpendicular condition

m1.m2 =-1
is the condition for perpendicular lines.. how is x=0 and y=0 perpendicular?

8 Answers

62
Lokesh Verma ·

hmm.. interesting...

i had to think :)

actually see the condition that is m1m2=-1 comes from another place:

Look closeley..

let the angle of first line from x axis is θ, 2nd line will have θ+Π/2

so slopes will be tanθ and tan(θ+Π/2)

tan(θ+Π/2) =cot{Π/2-(θ+Π/2)}
=-cotθ

thus, m1m2 = tanθ . -cotθ = -1 (but only when both tanθ and cotθ are defined....

So ur doubt is genuine... and the condition holds only when θâ‰
0 or Î /2... (nÎ /2) !!!!

So, technically (if one looks very very closely), in all such problems we should solve separately for these cases! (but we generally dont... and my best guess is that u wont be penalised for leaving this case!) and almost always u will get the right answer :)

I hope this makes it a bit clear?

1
Kumar Saurabh ·

hmm.. i understand...

i never had think of the proof of m1m2 as -1 ... goodd for that

thanks dude..

33
Abhishek Priyam ·

Maths doesnot capture ∞ but gives a hint of it. In this case m1=0 m2=∞ so m1m2 is indeterminate form so limit has 2 be calculated here also maths give us a hint.

1
Kumar Saurabh ·

hmm.. good comment abhishek :)

11
Mani Pal Singh ·

SIR JEE PERFECTLY ANALYSED
but mein iss mein thodi apni taang addana chahta hon :P

i was wondering y this could not be solved with the help of limits so let me try!!!!!

Let the slope of the line at the x axis be

Taking the R.H.L
Limit tan(0+h)
x--->0
and the line perpendicular be
Limit tan(pi/2+h)
h---->0

we know that m1 X m2 =-1

so

Limit tanh X( -coth)=-1 (∞ X 0)
h---->0

we can write this equation as

Limit tanh X (-1/tanh)=-1
h---->0
so we get

-1=-1

HENCE PROVED

SIMILARLY WE CAN CHECK ITS L.H.L AND WE WILL FIND THAT IT ALSO SATISFIES THE EQUATION

SO LIMIT EXISTS

HENCE PROVED

(CORRECT ME IF WRONG !!!!!!!!!!)

11
Mani Pal Singh ·

SIR WHAT ABOUT THIS APPROACH???

62
Lokesh Verma ·

see this approach is ok

but the bigger question is why do we not consider the case when m1=0 !!

that we should ideally do seperately!

24
eureka123 ·

Warning:I could be wrong with the approach

Consider I quadrant as a pair of lines
=> Combined eqn of pair of lines :0.x2 +xy+0.y2=0

for pair of lines to be perpendicular:
coeff (x2)+coeff( y2)=0
which is true........

so x=0 and y=0 are perpendicular to each other.

Your Answer

Close [X]