A Quadratic within a Quadratic
Example
A quadratic within a quadratic
Solve (x2 + 2x)2 - 11(x2 + 2x) + 24 = 0.
Solution
Note that x2 + 2x and (x2 + 2x)2 appear in the equation. Let a
= x2 + 2x and
substitute.
|
|
|
a2 - 11a + 24 |
= 0 |
|
|
|
|
(a - 8)(a - 3) |
= 0 |
Factor. |
a - 8 |
= 0 |
or |
a - 3 |
= 0 |
|
a |
= 8 |
or |
a |
= 3 |
|
x2 + 2x |
= 8 |
or |
x2 + 2x |
= 3 |
Replace a by x2 + 2x. |
x2 + 2x - 8 |
= 0 |
or |
x2 + 2x - 3 |
= 0 |
|
(x - 2)(x + 4) |
= 0 |
or |
(x + 3)(x - 1) |
= 0 |
|
x - 2 = 0 |
or |
x + 4 |
= 0 |
or |
x + 3 |
= 0 |
or |
x - 1 |
= 0 |
x = 2 |
or |
x |
= -4 |
or |
x |
= -3 |
or |
x |
= 1 |
Check. The solution set is {-4, -3, 1, 2}.
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