Categories
Geometry

Proof: Diameter is the Shortest Curve that bisects circular area

As shown in the figure below: (please scroll down a bit to see the figure) AB’ is one diameter of the circle, and a blue curve from A to B is supposed to bisect the area of the circle. We see from the figure below:

Length of the blue curve AB is greater than: AE + EB = AE + EB’

which is certainly longer than AB’; AB’ is the diameter.

The blue curve is obvious the focus. (Note when blue curve AB is mentioned, we mean the “curved one”: it curves around, not going straight from A to B.) Besides, point E is where the blue curve intersects with CD; and CD is one diameter.

So we have proved the claim that any curve bisecting the circular area got to be longer than the diameter.

Stay with us for one more minute. Let the construction-proof process be revealed to you, as follows.

Referring back to the figure. Let us start from the circle and the blue curve only (imagine all other lines and points disappear; now we draw them step by step). Connect the two endpoints A, B of the blue curve by a line segment AB. Then draw the diameter CD // AB (i.e. line CD is parallel to line AB).

Take point O (the midpoint of CD), and then passing A and O, let another diameter AB’ be drawn.

For completing the proof , two arguments are required:

(1) The blue curve has at least one intersection point with  diameter CD (thinking it: if the blue curve resides at only one side of CD, then that curve cannot divide the circular area evenly into two parts with equal area; therefore, any curves that bisects the circular area must intersects diameter CD). Now suppose the intersection point is E.

(2) B and B’ are symmetric to the diameter CD therefore EB = EB’ (trying justifying it using the property of circles and parallel lines).

The rest of the proof is straightforward (and intuitive).

Categories
Uncategorized

Pairing up with a Perfect Match

Math starts from the simple and goes a long way.

In this post, we will start from “pairing up” like 1 + 4 = 2 + 3 = 5 but there is a long way to go so that we learn the sets, working on the sets, pairing-up, observing and developing conditions for a perfect pair-up.

Pairing up for a Perfect Match

 

Categories
Junior Math (G9 and under) Skills in Math Contests

Sample Questions for Gauss Contests


Questions chosen from previous Gauss contests
Gauss contests are organized by the Centre of Education for
Math and Computing, University of Waterloo


Problem 1

In the addition shown, P and Q each represent single digits, and the sum is 1PP7. What is P + Q?

(A) 9 (B) 12 (C) 14 (D) 15 (E) 13

Problem 2

In the right-angled triangle PQR, we have that PQ = QR. The three segments QS, TU and VW are perpendicular to PR, and the segments ST and UV are perpendicular to QR, as shown. What fraction of triangle PQR is shaded?

(A) 3 ⁄ 16 (B) 3 ⁄ 8 (C) 5 ⁄ 16 (D) 5 ⁄ 32 (E) 7 ⁄ 32

Problem 3

A box contains a total of 400 tickets that come in five colors: blue, green, red, yellow, and orange. The ratio of blue to green to red tickets is 1 : 2 : 4. The ratio of green to yellow to orange tickets is 1 : 3 : 6. What is the smallest number of tickets that must be drawn to ensure that at least 50 tickets of the same colour have been selected?

(A) 50 (B) 246 (C) 148 (D) 196 (E) 115

Problem 4

Greg, Charlize, and Azarah run at different but constant speeds. Each pair ran a race on a track that measured 100 m from start to finish. In the first race, when Azarah crossed the finish line, Charlize was 20 m behind. In the second race, when Charlize crossed the finish line, Greg was 10 m behind. In the third race, when Azarah crossed the finish line, how many metres was Greg behind?

(A) 20 (B) 25 (C) 28 (D) 32 (E) 40

Problem 5

In right-angled, isosceles triangle FGH, segment FH = √̅8. Arc FH is part of the circumference of a circle with centre G and radius GH. The area of the shaded region is

(A) π – 2; (B) 4 π – 2 (C) 4 π – (1 ⁄ 2) √̅8 ; (D) 4 π – 4 (E) π – √̅8

Categories
Math for Early Age /Elementary (G5 under)

On the Patterns -Elementary

★ On the Patterns

 

Number Patterns * Numbers, Colors and Stars

Taking a look at the following chart.

1

2

★3

4

5

6

7

8

9

10

11

12

13

★14

15

16

17

18

19

20

21

22

23

24

★25

26

27

28

29

30

31

32

33

34

35

★36

37

38

39

40

41

42

43

44

45

46

★47

48

49

50

51

52

53

54

55

56

57

★58

59

60

The following questions are for everyone! Whether you are students in elementary schools (grade 3 and up), or in middle schools; or just parents who are tutoring your child; here are the challenges!

First, let us look up to the stars:

(1) What pattern is followed for the placement of stars ?

(2) Each number besides the star is increased by ____. (Fill in the blank)

(3) Following this pattern, the three more numbers that are besides and that comes after 58 are: __ , __ , and __.

(4) Now go back to the chart, and let us look at the colored cell (those colored by yellow)

(5) What can you do to follow the yellowed-colored cell? Please color the numbers on the chart by continue the pattern that you discovered.

(6) What are the common features of these yellow cells? Color some new cells, and explain how, by coloring these cells, you have followed and extended the pattern which is already in the chart.

Take a moment to think. You can answer these questions!

Categories
Algebra

Power Mad

Continue reading “Power Mad”
Categories
Uncategorized

Hello world!

Welcome to WordPress. This is your first post. Edit or delete it, then start writing!

Categories
Geometry Math BASICS Numbers

Protected: The (3-4-5) Pattern for Pythagorean Triplet [Thinking-of sides of a right triangle]

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Categories
Math BASICS Numbers

Number Sense – Activity: Tsunami Numbers in the News

About a decade ago, there was a great Tsunami happening in Asia.
What do you know about the Asian tsunami?

Read through the article first. Use the following numbers to fill in the blanks in the story.Think about which numbers make sense.

500  20  8,000;  2004  110,000  30,000  9.0

A tsunami triggered by a very large earthquake off the coast of the
Indonesian island of Sumatra on December 26, ____, has left
more than 150,000 people dead and millions homeless. Countries hit hardest by the disaster include
Sri Lanka, Indonesia, India, Thailand, and the Maldives. Almost 75% of the deaths occurred in
Indonesia, estimated at ____. Sir Lanka was second highest with about 20% of the estimated deaths, or
______ people lost that day. The rest of the deaths, approximately ____, occurred in the other nine
countries affected by the tsunami.
The ____ foot wall of water, higher than a two-story building,
swallowed entire villages. The tsunami waves were not only very high, they moved at a much faster speed
than normal. These waves were comparable in size to those you see on some of the surfing movies;
but those waves travel at 30 miles an hour, and the tsunami waves
were moving more than fifteen times as fast at ____ miles an hour.
The velocity of the force is what caused the destruction—a massive force that swept away everything in its path.

The earthquake causing this Tsunami was a destructive earthquake measuring ______ on the Richter scale,
the fourth worst earthquake in recorded history. Earthquakes are measured on a Richter scale that has
a range from 0 to 12; a 6.0 on the scale is a pretty bad earthquake.

(Story constructed from January 2005 news reports)

Categories
Geometry Math for Early Age /Elementary (G5 under)

The symbol π — where does it come from?

Where does the symbol π come from?

In 1652, William Oughtred used π to refer to the periphery of a circle (in his expression, the ratio of circumference-to-diameter of a circle is π ⁄ δ, the latter referring to diameter).

In 1665, Jonh Wallis used a Hebrew letter mem(mem) to equal the ratio of one-quarter of circumference to diameter of a circle. (This letter plays the role as of “M” in Latin alphabets, but look how close its shape resembles a quarter of a circle, as well as the Greek letter pi !)

In 1705, William Johns used π to represent the ratio of circumference-to-diameter of a circle (believed to be first use with exactly same meaning as in today) .

From 1736, Leonard Euler, both famous and a prolific writer in mathematics works, spread the use of π in his publications.

Counting from the first relevant use, the symbol π has already had a history of more than 360 years!

Categories
Numbers Uncategorized

Complete Numbers in Fraction Equations

The formula on our face page of “amazing numbers” is rather interesting:
1 – (1 ⁄ 28) = (1 ⁄ 2) + (1 ⁄ 4) + (1 ⁄ 7) + (1 ⁄ 14)

The point of interest is that: if you look at all divisors of 28: they are 1,2,4,7,14,28; with the exception of 28 which is itself, all divisors have appeared in this formula, and they appear in the form of so-called “unit fraction”, where numerator is 1. So (1 ⁄ 2), (1 ⁄ 4), etc. are all unit fractions.

Indeed, we present a fraction equation to make it a bit unusual, but there is a low-pitch but straightforward ways to present number 28. We have that:
28 = 1 + 2 + 4 + 7 + 14
To get to the earlier fraction form, just divide every term by the number 28.

The smallest complete number is 6 (=1+2+3), 28 is the 2nd complete number, and after that, you will not see a complete number until 496. So complete numbers are rare among all positive whole numbers.

Complete numbers 6 also has a nice fraction form, as:
1 – (1⁄6) = (1⁄2) + (1⁄3)

Categories
Numbers

Prime numbers

Prime numbers are those that have 1 (one) and itself as the only two divisors. Examples of primes are 2, 3, 5, 7, 11. None of 4, 6, 9 is a prime since 4 = 2 × 2, 6 = 2 × 3, and 9 = 3 × 3.

If a number greater than one is not a prime, then it is a composite number, and can be factored into the product of primes — called prime factorization. We have given the prime factorization of 4, 6, 9 as above. For a couple of more examples:

12 = 2 × 2 × 3

36 = 2 × 3 × 3 × 3

28 = 2 × 2 × 7

So all natural numbers are divided into three classes: the number 1, the prime numbers, and the composite numbers.

A bonus point: π, besides representing in a circle, the ratio of circumference to diameter, also stands for a special function related to prime numbers. Function π(x) — for every integer x, represent the number of primes less than or equal (i.e. not exceeding) x. For example, we have:

π(2) = 1, π(3) = 2, π(10) = 4, π(20) = 8 etc.
[To find why π(10) = 4, recall the 4 prime numbers not exceeding 10: they are 2,3,5, and 7.]