Monday, March 17, 2014

find the equation of the parabola with vertex on the line y=x, axis parallel to Ox, and passing through (6,-2),(3,4).analytic geometry,. conic...

The general equation of a parabola with an axis parallel
to Ox is (y - y0)^2 = 4a(x - x0), where the vertex is (x0,
y0).


Here the vertex lies on the line y = x, so we can
write it as


(y - c)^2 = 4a(x -
c)


The parabola goes through the points (6 , -2) and (3 ,
4). We get two equations to solve for a and c.


(4 - c)^2 =
4a( 3 - c) and ( -2 - c)^2 = 4a(6 - c)


(4 - c)^2 = 4a( 3 -
c)


=> 16 + c^2 - 8c = 12a - 4ac
...(1)


( -2 - c)^2 = 4a(6 -
c)


=> 4 + c^2 + 4c = 24a - 4ac
...(2)


(1) - (2)


=> 12
- 12c = -12a


=> c - 1 =
a


=> c = a +
1


Substituting in
(2)


=> 4 + (a + 1)^2 + 4(a + 1) = 24a - 4a(a +
1)


=> 4 + a^2 + 1 + 2a + 4a + 4 = 24a - 4a^2 -
4a


=> 5a^2 - 14a + 9 =
0


=> 5a^2 - 9a - 5a + 9 =
0


=> a(5a - 9) - 1(5a - 9) =
0


=> (a - 1)(5a -
9)


=> a = 1 and a =
9/5


c = 2 and c =
14/5


The equation of the parabola is (y -
2)^2 = 4(x - 2) and (y - 14/5)^2 = (36/5)(x -
14/5)

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