Author: jonah.luo

Categories
Geometry /Shapes

Find the Center of a Circle — Do you know how to do it?

Using a compass, you can draw a circle at any place, with any radius.

Now let’s reverse the problem. Given a circle, do you know how to find its center? (Of course, once the circle is found, there shall be no problem at all to tell its diameter, or radius.) You only see the circle itself, there is no explicit indication on where the center is.

In general, you can find the center using any convenient method, including copy-and-paste the circle onto a paper, and then fold it.

In particular (from classical Euclidean geometry), where it’s required to do so with a ruler (with which you are allowed to draw lines and line segments only) and a compass (with which you are allowed to draw circles only).

If you attempt to solve this problem with a ruler and a compass, then you are required to know how to make a perpendicular bisector. Do you know how to do it?

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. This is described as follows.

Function π(x) — for integer x, represents the number of primes less than or equal (i.e. not exceeding) x.

 

For example,

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.]

Categories
Math BASICS Numbers

Number Sense – Activity: Tsunami Numbers in the News

About two decades 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
Math for Early Ages (G7 and under) Math Modelling

cut a pizza into 11 equal slices, exactly evenly

What is the easiest way to cut a pizza into 11 equal slices?

The step-by-step solution is:

  • Take a wrist watch.
  • Position the watch hands to noon (12:00 exact), and put in the center of pizza.
  • Cut in the direction of watch hands.
  • Advance the watch until next overlapping of its hands…
Categories
Geometry /Shapes

3D objects with 3 views from top, front and side

For a 3D objects, given three views to you: one from top, one from front, and one from side, can you imagine what the original 3D objects looks like?

The question is not posed to a mechanic engineer, it would be trivial in that case. The question is raised to get a junior middle student to think a bit.

For a cylinder one of the three views is a circle, and the other two views are rectangles. For a cone one of the views is a circle, and the other two views are triangles. What if the three views given are a circle, a rectangle and a triangle? Can you figure out the original shape?

Categories
Math Contests - Problems and Discussion

Introduction to Canadian Math Contest

Have you heard about the Canadian Math Contests? It is certainly for those talented in math and logic to show their ability, and of course, good opportunities for those who love challenges.

For a student to participate in a contest, talk to the school he attends, or talk to us (Jonah’s Math Corner). Be sure enrolled in a middle school located in Canada (a student has to participate through a school or an agent in the country he/she currently studies at).

Please check the grades eligibility too – if it says grade 9, then all students in grade 10 and higher cannot write that contest. A general rule is students in lower grade are allowed to participate in contests for higher grade, but higher grade students CANNOT write contests for lower-grade.

For students wish to participate and challenge, it worths to have in mind the following Contest series: (apart from the Popular Math Contests – which include Math Kangaroo for grades 1 – 12, all other math contests are for middle school students)

1) Popular Math Contests: Math Kangaroo, started in Europe and becoming popular across world, is organized by Math Kangaroo company. Their are 6 age groups and each group bracket two grades, like grades 1-2, grades 3-4, .. .. According to the organizer’s claim, the purpose of the contest is to challenge students in a playful setting; instead of academic, it aims at developing mental powers and flexibility.

2) Junior math contests (have to be under grade 9 to participate): the most popular is the Gauss contest for grades 7 & 8, organized by U. of Waterloo. Gauss is very popular that some school teachers enrolled all of their students to participate. If you live in Alberta, then both U. of Calgary and U. of Alberta have set up challenges for Junior students.

3) Mid-level and High-school level Contests — the main stage for the middle school math challenge. University of Waterloo has organized 3 series respectively for grade 9, 10 and 11. The contests questions appears typically in multiple-choice format.

Series I: PCF contest: Pascal for grade 9, Cayley for grade 10 and Fermat – for grade 11.

Questions in this series appear in multiple-choice format.

Series II: FGH contest: Fryer – for grade 9, Galois – for grade 10 and Hypatia – for grade 11.

No multiple-choice question. All answers have to be worked out by participants. In any of these series, there will be two types of questions, one will ask participants for answers only, and the other type will ask students to write full solutions. Each contests is 75 minutes long.

Series III: CIMC – Canadian Intermediate Math Contests (for grade 9/10) and CSMC – Canadian Senior Math Contests (for grade 11/12). The format is similar to FGH, however, the level of challenge is in general higher, and students are provided with 2 hours time to answer.

4) Selective Contests for those best talents in math: Canadian Math Society have a challenge series for those who want to participate in Math Olympiad. The first step is the COMC (Canadian Open Math Challenge). It is selective, so only COMC is open to students; all following contests in the series, like CMO (Canadian Math Olympiad) and IMO (International Math Olymiad), are by-invitation ONLY.

5) Other Contests: besides those mentioned, we occasionally will register students in Regional Math League of Canada or US – some contests are online only – save students the cost and time to travel.

Jonah’s math corner can serve as a site to supervise students to write the math contests. If you have any question, just talk to us.

Categories
Math Contests - Problems and Discussion Math Junior (G9 and under)

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
Geometry /Shapes Math BASICS Numbers

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

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Categories
Algebra Numbers

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

What’s so special about number 8208?

The seemingly trivial, innocent number 8208 actually has something special. It is a member of a niche class of numbers called narcissistic numbers.

What’s so special?

First note it has 4 digits. If we raise every digit 8, 2, 0, 8 to the 4th power, and then add them together:

84 + 24 + 04 +84 = 8208

(To find 84, just think 84 = 212 = 4 x 210 = 4096)

Now read the four digits of the total. It’s 8, 2, 0, 8 again!

See the point? All the digits of the number are applied to generate that same number. Repeat the same procedure once more, you still get the same number. Seems this number loves itself so its ego is repeated. — That speaks for its name: narcissistic.

 

The 3-digit number 153 has similar property.

Since it has 3 digits, we raise every digit 1, 5, 3 to the 3rd power, and add them together!

13 + 53 + 33 = 153

Another time to witness the magic!

** **

Any one-digit number belongs to the class of narcissistic numbers. Reason like this: (if you raise everything to exponent one, it is just itself); this is plain yet may be too trivial to be interesting.

And tell you another secret. There is NOT ANY 2-digit number that is narcissistic (we assume that tens-digit cannot be zero: it has to be between 10 and 99). Some may be close (almost narcissistic!): for example

32 + 52 = 34;  72 + 52 = 74  (note 34 is just one off from 35)

That’s it. Happy reading!