Quadratic Equations

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September 19th 2008, 01:54 PM
lightningstab714

Quadratic Equations

Find the smallest possible value of the quantity $x^2 + y^2$ under the restriction that $2x + 3y = 6$

How would I go about solving this? Please help.
September 19th 2008, 01:58 PM
mr fantastic

Quote:

Originally Posted by lightningstab714

Find the smallest possible value of the quantity $x^2 + y^2$ under the restriction that $2x + 3y = 6$

How would I go about solving this? Please help.

$2x + 3y = 6 \Rightarrow x = \frac{6 - 3y}{2}$ .... (1)

Substitute (1) into $L = x^2 + y^2$ and find the value of y that minimises L. Substitute this value into L to get the minimum value of L.
September 19th 2008, 02:24 PM
lightningstab714

Thanks.
September 19th 2008, 06:14 PM
pankaj

Here are two non calculus approaches to your problem:

Geometrically, $x^2+y^2$ denotes square of distance of the point $(x,y)$ from $(0,0)$ therefore,you have been asked to find the shortest distance between $(0,0)$ and the line $2x+3y=6$ in 2D space which is the perpendicular distance of 2x +3y=6 from (0,0) and which can be found from the formula
$p=\frac{|ax_{1}+by_{1}+c|}{\sqrt{a^2+b^2}}$
$=\frac{|2(0)+3(0)+6|}{\sqrt{2^2+3^2}}$
$=\frac{6}{\sqrt{13}}$ .Therefore answer should be $<br /> \frac{36}{13}$

Another method:

Put $x=r\cos\theta;y=r\sin \theta$ where $r=\sqrt{x^2+y^2}$

Plugging in $2x+3y=6$ ,we get

$r=\frac{6}{2\cos \theta +3\sin \theta}$

Now r will be minimum when denominator will be maximum.

Also $,-\sqrt{a^2+b^2} \leq a\cos\theta+b\sin\theta \leq\sqrt{a^2+b^2}$

Therefore, $r_{min}=\frac{6}{\sqrt{2^2+3^2}}=\frac{6}{\sqrt{13 }}$
and hence $x^2+y^2=\frac{36}{13}$