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Problems 2.21, 2.22, 2.23

RoseWind [281]

Statements can be proved by contrapositive, contradiction or by induction.

2.21 and 2.23 are proved by contrapositive
2.22 is proved by induction

2.21: If $n^3$ is even, then n is even (By contrapositive)

The contrapositive of the above statement is that:

If n is odd, then $n^3$ is odd

Represent the value of n as:

$n = 2k + 1$ , where $k \ge 0$

Take the cube of both sides

$n^3 = (2k + 1)^3$

Expand

$n^3 = 8k^3 + 6k^2 + 6k + 1$

Group

$n^3 =[ 8k^3 + 6k^2 + 6k] + 1$

Factor out 2

$n^3 =2[4k^3 + 3k^2 + 3k] + 1$

Assume w is an integer; where:

$w =4k^3 + 3k^2 + 3k$

So, we have:

$n^3 =2w + 1$

The constant term (i.e. 1) means that $n^3$ is odd.

Hence, the statement has been proved by contrapositive.

i.e. If n is odd, then $n^3$ is odd

2.22 $3n + 4$ is even, if and only if n is even

We have: $3n + 4$

Assume that: $n = 2k + 2$ for $k \ge 0$

So, we have:

$3n + 4 = 3(2k + 2) + 4$

Open bracket

$3n + 4 = 6k + 6 + 4$

$3n + 4 = 6k + 10$

Factorize

$3n + 4 = 2(3k + 5)$

The factor of 2 means that $3n + 4$ is even.

Hence, $3n + 4$ is even, if and only if n is even

2.22: $s \ne -1$ and $t \ne -1$ , then $s + t + st \ne -1$

To do this, we prove by contrapositive.

The contrapositive of the above statement is:

If $s = -1$ and $t=-1$ , then $s + t + st = -1$

We have:

$s + t + st = -1$

Substitute the values of s and t in: $s + t + st = -1$

$-1 -1 -1 \times -1 = -1$

$-1 -1 + 1 = -1$

$-1 = -1$

Hence, by contrapositive:

If $s = -1$ and $t=-1$ , then $s + t + st = -1$

Read more about proofs at:

brainly.com/question/19643658

7 0

3 years ago

What is x equal to: 3x=1

Verdich [7]

Answer:

x = $\frac{1}{3}$

Step-by-step explanation:

Step 1: Solve

We can divide 3 from both sides to get "x" by itself

$3x=1\\\frac{3x}{3} =\frac{1}{3} \\x = \frac{1}{3}$

Therefore x = $\frac{1}{3}$

5 0

3 years ago

Read 2 more answers

Why is it important to include a scale when displaying a box plot?

KIM [24]

Because then it would be easier to get an answer for a problem

6 0

3 years ago

At a video arcade action video games usually cost $12.50. This week they are on sale for 30% off. What is the sale price of the

Juliette [100K]

Answer: 8.75

Step-by-step explanation:

12.50 x 30% or .03

= 3.75

3.75 - 12.50

= 8.75

5 0

3 years ago

Read 2 more answers

The Surface Area of a square pyramid is 3600 ft. The slant height is 80 feet and the base is 20 feet. At most how many 1 foot cu

amm1812

Answer:

10,583 flexible cubes

9915 rigid cubes

Step-by-step explanation:

The equation for the surface area of a square pyramid is area of base plus perimeter of base multiplied by the slant height

Surface area = A + 1/2·p·s

The volume of the pyramid = 1/3×A×h

Where:

A = Area of the base = 20 × 20 = 400 ft²

h = The height of the pyramid = √((slant height)² - ((base side length)/2)²)

h = √(80² - (20/2)²) = 30·√7

The volume of the pyramid = 1/3*400*30·√7 = 4000·√7 ft³ = 10,583.005 ft³

If the cubes are flexible, approximately 10,583 cubes

For rigid cubes we have

Given that the height of the pyramid = 30·√7

The slope of the pyramid = 30·√7/10 = 3·√7

So an increase in height of 1 foot gives a reduction in width of 2/(3·√7)

The bottom can hold 400 cubes

The next layer can hold 19×19 = 361

3rd 361

4th 361

5th 324

6th 324

7th 324

8th 324

9th

Continuing, we get a total of about 9915 rigid cubes in the pyramid.

3 0

3 years ago