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I have to find out the vertex of a parabola given by:

$$ 9x^2-24xy+16y^2-20x-15y-60=0 $$

I don't know what to do. I tried to bring it in the form:

$$ (x-a)^2 + (y-b)^2 = \dfrac {(lx+my+n)^2} {l^2+m^2} $$

but failed in doing so. Is there any other way to solve the problem? Or maybe you could help me bring the equation in the above form.

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find its reduced equation in a rotated frame. –  metacompactness Jul 31 '13 at 15:13
    
How to do that? I am new to this rotation business. –  Parth Thakkar Jul 31 '13 at 15:14
    
    
@vadim123, thanks! –  Parth Thakkar Jul 31 '13 at 15:17
    
Draw a picture! Not for me, for you. –  Will Jagy Jul 31 '13 at 20:23

2 Answers 2

up vote 1 down vote accepted

Think of the standard equations of a parabola you know $y=x^2$ or $y^2=4ax$ - something squared = something linear, and the squared quantity and the linear quantity represent axes at right-angles to one another.

The vertex occurs where the squared quantity is equal to zero, ie on the axis of symmetry.

Now notice that the equation can be rewritten as $$(3x-4y)^2=5(4x+3y+12)$$

Check that the axes implied by this form are perpendicular.

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Hint: At the vertex of a parabola, the change of slope of tangent attain maximum or minimum, i.e. $y'''=0$.

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I mean "the change of slope of tangent attain maximum or minimum". –  ᴊ ᴀ s ᴏ ɴ Jul 31 '13 at 15:21
    
Or $y'''$ undefined. This is a very roundabout way of solving a straightforward problem. –  vadim123 Jul 31 '13 at 15:45

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