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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: find min value of x^2 + 2xy + 3y^2 -6x - 2y
Forum Index -> Differential Calculus like the article? email it to a friend.  
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umang (229)

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x,y belongs R

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computer001 (1847)

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is it -11??

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umang (229)

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yes it is
pls explain !

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computer001 (1847)

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well c it lik this...wrt x y is const..
so lets call it f(x) where coeff include y...
now find f'(x)=0 u get rel x=3-y chk for f"(x) also
now v get tht @ x=3-y f(x) has min value= 2y^2+4y-9
(which u get by subst x in 1st eqn  i.e f(x))
now v have to find min val of this...
so diff 2y^2 +4y-9=0 u get min value @ y=-1
so min val is -11
now if u r askin y not i initiall do as f(y) ok do the same way..
u get min value @ 3y=1-x in tht case
and if u follow same procedure u again get -11

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umang (229)

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its confusing !
pls explain in a simple manner !!
thank you !

Umang
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computer001 (1847)

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welll...
u got 2 wait till some1 else comes along

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umang (229)

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hey !
i got it !
u hav done a lot of typing mistake !!!
thanks !!

Umang
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computer001 (1847)

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now wait a minute...cud u point out a few???

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umang (229)

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@ x=3-y f(x)
u wrote @ instead of 2.
but anyways, thanks !!!
u appearing for JEE 2008 ?
got ur admit card ?

Umang
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computer001 (1847)

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no mate its correct only...
this is wat i have said "now we get that at x=3-y , f(x) has min value"...
yeah i am...got my admit card a wk ago

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umang (229)

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oh sorry !
i understood !
now ur method is completely clear to me !!!

Umang
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little_genius (274)

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let us first fix the value of y and then vary x to find the minimum value of func. for that fixed value of y... we get x= 3-y....
so the min value for that fixed y is
2y^2 +4y -9.....
 
we had kept y fixed......now we vary y too to get the least value among all values of y....
so we get at y= -1 , the expression has the min value for all x ,y...........
thus min val= -11......
 

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