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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