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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: Parabola!!Qeustion:
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AJ1991 (216)

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Write the equation of the common normal to y^2=4ax and x^2=4ay!!

<TABLE CELLSPACING="1" CELLPADDING="1" BORDER="0">
<TR><TD>


<DIV ALIGN="right">Glitter Graphics</DIV></TD></TR></TABLE>







    
raulrag009 (1194)

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 it is y=2x-12a,  y=-x+3a
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AJ1991 (216)

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hey man, can u show the working plz..
Ill give u good rating if it is good.

<TABLE CELLSPACING="1" CELLPADDING="1" BORDER="0">
<TR><TD>


<DIV ALIGN="right">Glitter Graphics</DIV></TD></TR></TABLE>







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rhd92781 (686)

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the common normal of both parabolas shud be normal to y=x also.
so its slope is -1.
now juss, y=mx-2am-am^3, put m=-1
x+y=3a

<TABLE CELLSPACING="1" CELLPADDING="1" BORDER="0">
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<DIV ALIGN="right">Animated Letters</DIV></TD></TR></TABLE>

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raulrag009 (1194)

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hey rhd92781
could u show ur steps
i did'nt understand ur concept man!!!!!!!!
 
 
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AJ1991 (216)

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hey yaar, you did'nt write anything??

<TABLE CELLSPACING="1" CELLPADDING="1" BORDER="0">
<TR><TD>


<DIV ALIGN="right">Glitter Graphics</DIV></TD></TR></TABLE>







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AJ1991 (216)

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hey raulrag,why dont' you show your steps...man,,?

<TABLE CELLSPACING="1" CELLPADDING="1" BORDER="0">
<TR><TD>


<DIV ALIGN="right">Glitter Graphics</DIV></TD></TR></TABLE>







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raulrag009 (1194)

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SEE man , I hav purposly changed one step in my working ..
keep seeing my steps, i'll notify it when i change it
 
 
Let the eqn of normal to the parabola y2=4ax  be
y=mx-2am-am3
 
since it is common to the other parabola
 
x2=4a(mx-2am-am3)
x2-4amx+8a2m+4a2m3=0
 
since this normal cuts the parabolas at three distinct points
m will have three distinct values
 
thus
D=0
16a2m2-4(8a2m+4a2m3)=0
16a2m2-32a2m-16a2m3=0
16a2m(m-2-m2)=0
 
thus u get
 
m=0  ,   m2-m+2=0  ............{here  as u can see discriminant for
                                            this eqn is negetive}
 
But if u take the eqn as
    
 
m^2-m-2=0.
then u get
 
m=-1,2
 
thus u will get ur correct answer
 
that is why iam getting a bit confused man
   
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AJ1991 (216)

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hey raulrag,how come the normal cuts the parabola in 3 distinct pts, that is not possible..

<TABLE CELLSPACING="1" CELLPADDING="1" BORDER="0">
<TR><TD>


<DIV ALIGN="right">Glitter Graphics</DIV></TD></TR></TABLE>







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raulrag009 (1194)

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hey sorry
that reason is not appropiate but steps are correct
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