1answer.
Ask question
Login Signup
Ask question
All categories
  • English
  • Mathematics
  • Social Studies
  • Business
  • History
  • Health
  • Geography
  • Biology
  • Physics
  • Chemistry
  • Computers and Technology
  • Arts
  • World Languages
  • Spanish
  • French
  • German
  • Advanced Placement (AP)
  • SAT
  • Medicine
  • Law
  • Engineering
Goryan [66]
1 year ago
6

If

4\sqrt{5}" align="absmiddle" class="latex-formula">, find the value of \sqrt{x} - \frac{1}{\sqrt{x} }.
Mathematics
1 answer:
Komok [63]1 year ago
5 0

Observe that

\left(\sqrt x - \dfrac1{\sqrt x}\right)^2 = \left(\sqrt x\right)^2 - 2\sqrt x\dfrac1{\sqrt x} + \left(\dfrac1{\sqrt x}\right)^2 = x - 2 + \dfrac1x

Now,

x = 9 - 4\sqrt5 \implies \dfrac1x = \dfrac1{9-4\sqrt5} = \dfrac{9 + 4\sqrt5}{9^2 - \left(4\sqrt5\right)^2} = 9 + 4\sqrt5

so that

\left(\sqrt x - \dfrac1{\sqrt x}\right)^2 = (9 - 4\sqrt5) - 2 + (9 + 4\sqrt5) = 16

\implies \sqrt x - \dfrac1{\sqrt x} = \pm\sqrt{16} = \pm 4

To decide which is the correct value, we need to examine the sign of \sqrt x - \frac1{\sqrt x}. It evaluates to 0 if

\sqrt x = \dfrac1{\sqrt x} \implies x = 1

We have

9 - 4\sqrt5 = \sqrt{81} - \sqrt{16\cdot5} = \sqrt{81} - \sqrt{80} > 0

Also,

\sqrt{81} - \sqrt{64} = 9 - 8 = 1

and \sqrt x increases as x increases, which means

0 < 9 - 4\sqrt5 < 1

Therefore for all 0 < x < 1,

\sqrt x - \dfrac1{\sqrt x} < 0

For example, when x=\frac14, we get

\sqrt{\dfrac14} - \dfrac1{\sqrt{\frac14}} = \dfrac1{\sqrt4} - \sqrt4 = \dfrac12 - 2 = -\dfrac32 < 0

Then the target expression has a negative sign at the given value of x :

x = 9-4\sqrt5 \implies \sqrt x - \dfrac1{\sqrt x} = \boxed{-4}

Alternatively, we can try simplifying \sqrt x by denesting the radical. Let a,b,c be non-zero integers (c>0) such that

\sqrt{9 - 4\sqrt5} = a + b\sqrt c

Note that the left side must be positive.

Taking squares on both sides gives

9 - 4\sqrt5 = a^2 + 2ab\sqrt c + b^2c

Let c=5 and ab=-2. Then

a^2+5b^2=9 \implies a^2 + 5\left(-\dfrac2a\right)^2 = 9 \\\\ \implies a^2 + \dfrac{20}{a^2} = 9 \\\\ \implies a^4 + 20 = 9a^2 \\\\ \implies a^4 - 9a^2 + 20 = 0 \\\\ \implies (a^2 - 4) (a^2 - 5) = 0 \\\\ \implies a^2 = 4 \text{ or } a^2 = 5

a^2 = 4 \implies 5b^2 = 5 \implies b^2 = 1

a^2 = 5 \implies 5b^2 = 4 \implies b^2 = \dfrac45

Only the first case leads to integer coefficients. Since ab=-2, one of a or b must be negative. We have

a^2 = 4 \implies a = 2 \text{ or } a = -2

Now if a=2, then b=-1, and

\sqrt{9 - 4\sqrt5} = 2 - \sqrt5

However, \sqrt5 > \sqrt4 = 2, so 2-\sqrt5 is negative, so we don't want this.

Instead, if a=-2, then b=1, and thus

\sqrt{9 - 4\sqrt5} = -2 + \sqrt5

Then our target expression evaluates to

\sqrt x - \dfrac1{\sqrt x} = -2 + \sqrt5 - \dfrac1{-2 + \sqrt5} \\\\ ~~~~~~~~~~~~ = -2 + \sqrt5 - \dfrac{-2 - \sqrt5}{(-2)^2 - \left(\sqrt5\right)^2} \\\\ ~~~~~~~~~~~~ = -2 + \sqrt5 + \dfrac{2 + \sqrt5}{4 - 5} \\\\ ~~~~~~~~~~~~ = -2 + \sqrt5 - (2 + \sqrt5) = \boxed{-4}

You might be interested in
Write 20 as a product of prime factors.
Aliun [14]

Answer:

1, 2, 4, 5, 10

Step-by-step explanation:

4 0
3 years ago
Read 2 more answers
8 Line in the xy-plane contains points from each of Quadrants II, III, and IV, but no points from Quadrant I. Which of the follo
iren2701 [21]

Answer:

The correct option is D.

Step-by-step explanation:

The slope of a line is the change in y with respect to x.

m=\frac{y_2-y_1}{x_2-x_1}

If the slope of a line is undefined it means it is a vertical line and a vertical line  can not passes through three quadrants. So, option A is incorrect.

If the slope of a line is 0 it means it is a horizontal line and a horizontal line  can not passes through three quadrants. So, option B is incorrect.

If the slope of a line is positive it means the value of y increases as x increases.

Since it is an increasing line, therefore after a certain period both x and y will positive. It means the line will passes through first quadrant. So, option C is incorrect.

If the slope of a line is negative it means the value of y decreases as x increases. It can passes through each of Quadrants II, III, and IV.

Therefore the correct option is D.

6 0
2 years ago
In choosing this year 327 voted musical and 115 voted drama. How many more students voted musical than drama
Lapatulllka [165]

Answer:

212 more student voted musical than drama .

Step-by-step explanation:

As given

In choosing this year 327 voted musical and 115 voted drama.

More students voted musical than drama = Total students voted for musical - Students voted for drama .

Putting values in the above

More students voted musical than drama = 327- 115

                                                                      = 212

Therefore 212 more student voted musical than drama .

5 0
3 years ago
A school band is selling raffle tickets for $2 each there are 2 groups selling tickets. Each group have 4 students the tacher gi
nordsb [41]

Answer:it’s bbb

Step-by-step explanation:

4 0
2 years ago
Use the figure to answer the questions below.
alina1380 [7]

Answer:

  • parallelogram
  • x = 5
  • 60 units

Step-by-step explanation:

A. Opposite sides are the same length, so the figure is a parallelogram.

__

B. Diagonals of a parallelogram cross at their midpoints, so ...

  9x +3 = 10x -2

  5 = x . . . . . . . . add 2-9x to both sides

__

C. The longer side is 12x = 12(5) = 60 . . . . units

5 0
3 years ago
Other questions:
  • Is 100,60,50 a right triangle
    14·1 answer
  • The questions is above ⤴​
    13·1 answer
  • n 1917 the cost of a first-class postage stamp was 3¢. In 1974 the cost for a first-class postage stamp was 10¢. What is the per
    7·1 answer
  • Which graph represents y=x-2
    6·2 answers
  • Answer please maths I’m begging you please it’s for a test
    10·1 answer
  • Rufus invest 10,000 in a savings account that pays 3.5 simple interest each year. If Rufus makes no withdrawal or deposits to th
    5·1 answer
  • Which of the following represents the probability that someone who
    9·1 answer
  • The leg of an isosceles triangle has a length 2x+4 and the base has length 3x. The
    9·1 answer
  • Find the slope of the given table
    6·1 answer
  • Consider a family that generates 20 loads of laundry in a week. based on the data in the table, what is the total number of peop
    9·1 answer
Add answer
Login
Not registered? Fast signup
Signup
Login Signup
Ask question!