inequalities

2007-11-24 7:17 pm
for the following case, find the ragr of x.
(x-6)^2 = 3|x-6|+10
(x-6)^2 < 3|x-6|+10
thx your help

回答 (2)

2007-11-24 8:22 pm
✔ 最佳答案
You need to set 2 cases for the range of value of X in order to solve the problem.

For x&gt;6
(x-6)^2 = 3(x-6) + 10 (since x-6 is positive when x&gt;6)
x^2 - 12x + 36 - 3x + 18 - 10 = 0
x^2 + 24x + 44 = 0
x = -2(rejected)or x= -22 (rejected)

For x&lt;6
(x-6)^2 = -3(x-6) + 10 (since x-6 is negative when x&lt;6)
x^2 - 12x + 36 + 3x - 18 - 10 = 0
x^2 -9x + 8 = 0
x=8 (rejected) or x=1

So,the root of the equation &quot;(x-6)^2 = 3|x-6|+10&quot; is x=1

Similarly
For x&gt;6
(x-6)^2 - 3(x-6) - 10 &lt; 0
x^2 + 24x + 44 &lt; 0
x &lt; -2 (rejected)or x &gt; -22
by combing x&gt;-22 and x&gt;6
the first range of x is x &gt; -22

For x&lt;6
(x-6)^2 + 3(x-6) - 10 &lt; 0
x^2 - 12x + 36 + 3x - 18 - 10 &lt; 0
x^2 -9x + 8 &lt; 0
x &lt; 8 or x &gt; 1
The second range of x is x &lt; 6
Range of values of x: -22 &lt; x &lt; 6
參考: myself
2007-11-25 4:43 am
Here is a simpler method:

For (x - 6)^2 = 3|x - 6|+10, note that (x - 6)^2 = |x - 6|^2
So, we have
|x - 6|^2 = 3|x - 6|+10
|x - 6|^2 - 3|x - 6| - 10 = 0
(Note that y^2 - 3y - 10 = 0 gives y = 5 or -2)
|x - 6| = 5 or - 2 (rejected, as absolute value always non-negative)
x - 6 = 5 or -5
x = 11 or 1

-------------------------------------------------------------------------------------------------
for (x-6)^2 &lt; 3|x-6|+10, we have
|x - 6|^2 - 3|x - 6| - 10 &lt; 0
(Note that y^2 - 3y - 10 &lt; 0 gives -2 &lt; y &lt; 5)
-2 &lt; |x - 6| &lt; 5
|x - 6| &lt; 5 (again, sbsolute value always non-negative)
-5 &lt; x - 6 &lt; 5
-1 &lt; x &lt; 11


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