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Find all values of  satisfying\(Find all values of $x$ satisfying \[ \sqrt{4x-3}+\frac{10}{\sqrt{4x-3}}=7. \] \)

 Jun 2, 2016

Best Answer 

 #1
avatar+23252 
+10

Solve     sqrt(4x - 3) + 10 / sqrt(4x - 3)  =  7

 

First note that the expression 4x - 3 is under the square root sign.

Since this value must be positive:      4x - 3  >=  0     --->     4x  >= 3     --->     x >= 4/3

 

To remove the denominator from the problem multiply each side by  sqrt(4x - 3):

     sqrt(4x - 3) [ sqrt(4x - 3) + 10 / sqrt(4x - 3) ]    =    sqrt(4x - 3) [ 7 ]

--->     sqrt(4x - 3) · sqrt(4x - 3)  +  sqrt(4x - 3) · 10  sqrt(4x - 3)    =    7· sqrt(4x - 3)

--->                (4x - 3)                  +                  10                            =    7 · sqrt(4x - 3)

--->                                     4x + 7   =   7·sqrt(4x - 3)

 

Square both sides:             [ 4x + 7 ]2   =   [ 7·sqrt(4x - 3) ]2  

                                   4x2 + 56x + 49   =   49(4x - 3)

--->                             4x2 + 56x + 49   =   196x - 147

Subtract 196x from both sides, add 147 to both sides:

--->                          4x2 - 140x + 196   =   0

Divide both sides by 4:

--->                            x2 - 35x + 49   =   0

Factor:                      (4x - 7)(x - 7)  =  0

So, either 4x - 7  =  0   --->    x = 7/4     or     x - 7  =  0     --->   x = 7

Since both answers are > 4/3, these are possible answer.

I'll leave the check for you ...

 Jun 2, 2016
 #1
avatar+23252 
+10
Best Answer

Solve     sqrt(4x - 3) + 10 / sqrt(4x - 3)  =  7

 

First note that the expression 4x - 3 is under the square root sign.

Since this value must be positive:      4x - 3  >=  0     --->     4x  >= 3     --->     x >= 4/3

 

To remove the denominator from the problem multiply each side by  sqrt(4x - 3):

     sqrt(4x - 3) [ sqrt(4x - 3) + 10 / sqrt(4x - 3) ]    =    sqrt(4x - 3) [ 7 ]

--->     sqrt(4x - 3) · sqrt(4x - 3)  +  sqrt(4x - 3) · 10  sqrt(4x - 3)    =    7· sqrt(4x - 3)

--->                (4x - 3)                  +                  10                            =    7 · sqrt(4x - 3)

--->                                     4x + 7   =   7·sqrt(4x - 3)

 

Square both sides:             [ 4x + 7 ]2   =   [ 7·sqrt(4x - 3) ]2  

                                   4x2 + 56x + 49   =   49(4x - 3)

--->                             4x2 + 56x + 49   =   196x - 147

Subtract 196x from both sides, add 147 to both sides:

--->                          4x2 - 140x + 196   =   0

Divide both sides by 4:

--->                            x2 - 35x + 49   =   0

Factor:                      (4x - 7)(x - 7)  =  0

So, either 4x - 7  =  0   --->    x = 7/4     or     x - 7  =  0     --->   x = 7

Since both answers are > 4/3, these are possible answer.

I'll leave the check for you ...

geno3141 Jun 2, 2016
 #2
avatar+129852 
+5

Let  √[4x - 3]  =  q^2

 

So we have

 

q^2  +  10/q^2   = 7      multiply both sides by q^2

 

q^4 + 10  = 7q^2

 

q^4 - 7q^2 + 10   = 0    factor

 

(q^2 - 5) ( q^2 - 2)  = 0

 

So either

 

q^2  = 5     or    q^2   =  2    .....so....

 

√[4x - 3]  = 5        square both sides

 

4x - 3   = 25     add 3 to both sides

 

4x = 28

 

x = 7

 

Or

 

√[4x - 3]  = 2     square both sides

 

4x - 3  = 4      add 3 to both sides

 

4x = 7

 

x = 7/4

 

 

Check  x = 7

 

√[4x - 3] +  10 / √[4x - 3] = 7  ??

 

√[4*7 - 3] + 10 / √[4*7 - 3]  = 7  ??

 

5 + 10/5  = 7      is true

 

Check  x  = 7/4

 

√[4(7/4) - 3] +  10 / √[4(7/4) - 3] = 7   ??

 

√[7-3] +  10 / √[7 - 3]] = 7  ??

 

2 + 10/2   = 7   is also true

 

So  x   =  7    or   x  = 7/4

 

 

 

cool cool cool

 Jun 2, 2016

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