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3 years ago

Last week, the price of gas was $2.25 per gallon. This week,

Mathematics

1 answer:

Sophie [7]3 years ago

3 0

Answer:

$2.25 + 4% = $2.34

It increased by 9 cents

Step-by-step explanation:

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If <img src="https://tex.z-dn.net/?f=x%20%3D%209%20-%204%5Csqrt%7B5%7D" id="TexFormula1" title="x = 9 - 4\sqrt{5}" alt="x = 9 -

Komok [63]

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

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

5 0

2 years ago

Find the surface area and the volume, help me pretty please

Dimas [21]

Answer:

The surface area is 13.08 and the volume is 3.14

Step-by-step explanation:

4 0

3 years ago

Find the values of x and y if (-x + 5, 1) = (-y, 2x - 5y).

MrMuchimi

Answer:

x = 8, y = 3

Step-by-step explanation:

Equating corresponding x and y coordinates , then

- y = - x + 5 ( multiply through by - 1 )

y = x - 5 → (1)

2x - 5y = 1 → (2)

Substitute y = x - 5 into (2)

2x - 5(x - 5) = 1 ← distribute and simplify left side

2x - 5x + 25 = 1

- 3x + 25 = 1 ( subtract 25 from both sides )

- 3x = - 24 ( divide both sides by - 3 )

x = 8

Substitute x = 8 into (1) for corresponding value of y

y = 8 - 5 = 3

7 0

3 years ago

Mia's work to find the slope of a trend line through the points (3, 10) and (35, 91) is shown below. Mia's Work Step 1 StartFrac

gogolik [260]

Answer:

Mia switched the numerator and the denominator.

Step-by-step explanation:

i got it right on edge 2020

and uh also because its y2 - y1 over x2 - x1, not x2 - x1 over y2-y1.

7 0

3 years ago

In triangle ABC, the side lengths are AB = 13, AC = 21, and BC = x. Write a compound inequality that represents the range of pos

Ipatiy [6.2K]

Answer:

7 < x < 34

Step-by-step explanation:

The answer must be greater than 7, else the two sides are less than or equal to the third. That triangle is impossible. The answer must also be less than 34, because then the two other sides would equal the third, making an impossible triangle.

4 0

3 years ago