It's A Square Deal

My math-a-magical powers are back.  In my January 23rd post, I demonstrated how to easily divide by the decimal .25.  Today let's look put on our magician's hat and learn how to square any two digit number that ends in 5.

In the number below, look at the digit in the tens place (3) and count up one more.  In this case when we count up, the 3 becomes a 4.  Now multiply the two numbers together, 4 × 3 which equals 12.  Next do 5² which is 5 × 5 = 25.  Therefore, the answer to 35² is 1,225.

Let's try another one.  Remember, it must be a two digit number that ends in 5 for the magic trick to work. This time let's start with 65 and follow the exact same procedure used above.

Did you get the right answer?  Have your students try this using all the two digits numbers that end in 5, beginning with 15 and finishing with 95.  Since all math is based on patterns, ask hem to carefully look for a pattern and describe it.  I think you will be very surprised at what you find!

 

The Power of Math Tricks

Math tricks will never make you a great mathematician, but in the eyes of some, you can be a fantastic math-a-magician. My college students love it when I show them a trick they can then take home to amaze and impress their peers, parents, children or the best yet - their spouses. 

Remember when I demonstrated how to easily multiply by 11 in the post The Eleventh Hour?  Or how about the trick of multiplying by 12 in Quick Times?   Here is a new one I recently showed my students.

First, let's look at a problem where a number is divided by the decimal .25. 

The above example is really 9 x 4 which is 36, but why is this true?  Hopefully your students know that .25 is equivalent to 1/4; so this problem can be reworded as  9 divided by 1/4.  As seen below, when dividing a whole number by a fraction, we find the reciprocal of 1/4 and then multiply which gives us the answer of 36.

 

Based on the sample above, anytime a problem requires dividing by .25, simply multiply by four to get the correct answer.  Try these without using a calculator or paper and pencil.





Instead of using the reciprocal to divide fractions, I teach my students that this is the "cross" method. Simply look at the original problem and cross multiply as seen in the illustration below. 



Fractions for the
Confused
and Bewildered

First multiply the bottom right denominator with the top left numerator. (4 x 9)  Next multiply the bottom left denominator with the top right numerator, (1 x 1) and you get an answer of 36.  When doing the fractions this way, there is no confusion on the students' part about which fraction to invert. If you would like a more details on how to divide fractions this way, go to the post entitled: Don't Flip!

If you are interested in other alternative ways to teach the four operations of fractions, you can check out the resource on your right.


By the way, the answers to the above problems are  a) 24    b) 80    c) 72    d) 380.    How did you do?

Parabola - The Arch Enemy?

I love to relate math to the real world with my students because that is the only way they will see the relevance.  Our family (son + his wife + three grandkids + husband) just returned from a trip to Orlando.  While driving home from Cocoa Beach, my daughter-in-law noticed a purple arch.  Being that my mind is always, always thinking about math, I informed her that it was a parabola with a negative slope.  My son, who is an engineer, starting talking about slope, and the two of us shared some equations such as the one for lines y = mx + b (much to the chagrin of the other travelers stuck in the car with us).

Mathematically speaking, a parabola is a two-dimensional, symmetrical curve or simply, a special curve shaped like an arch.  All parabolas are vaguely “U” shaped, and they have a highest or lowest point called the vertex.  The vertex is the place the parabola makes it sharpest turn.  Any point on a parabola is at an equal distance from a fixed point (the focus), and a fixed straight line (the directrix). Parabolas may open up or down and may or may not have x-intercepts, but they will always have a single y-intercept. Those that open up have a positive slope (they form a smile) and those that open down have a negative slope (they form a frown).  We always get a parabola when we graph a quadratic equation, an equation that contains a variable that is squared such as y² = 20x or x² - 9y = 0.

Now that all of this is as clear as mud for many of you, let's look at some parabolas in real life.  Yes, they are out there!  Can you identify the ones below?

Did the last picture stump you?  Well, it would unless you were from Los Angeles.  It is the

Encounter Restaurant, atop Los Angeles International Airport’s landmark Theme Building.

Other real life examples include....

1) Throwing or Kicking a Ball - If you throw a baseball, kick a soccer ball, shoot an arrow, fire a missile, or throw a stone, it will arc up into the air and come down again following the path of a parabola! (Except for how the air affects it.) The next time you watch a football being thrown from the quarterback to a receiver, think of a parabola.

2) Roller Coasters that arc up and down and sometimes around - the one ride I avoid! When a coaster falls from the peak (vertex) of the parabola, it is rejecting air resistance, and all the bodies are falling at the same rate. The only force here is gravity. Most people (I am NOT included) enjoy or get a thrill out of parabolic-shaped coasters because of the intense pull of gravity.

3) Reflectors - Parabolas are also used in satellite dishes and flashlights. In satellite dishes it helps reflect signals that then go to a receiver, which interprets the signals and shows satellite-transmitted channels on your television. In flashlights, car headlights and spotlights, the parabolic shape helps reflect light. Notice the beam of light coming from the flashlight on your right. See how the light appears to be in the shape of a parabola?

4) Suspension Bridges such as the Golden Gate Bridge, the Brooklyn Bride, the Washington Bridge, etc.  Suspension bridges are capable of spanning long distances and actually are the only type of bridge to span the longest distances possible for a bridge. This is because the shape of the suspension bridge is actually one of the most stable structures there is. In the image of above, can you see how the cables form parabolas?

So now you know parabolas are everywhere even when you are playing ports, watching T.V., riding a roller coaster at your favorite theme park or going cross a suspension bridge.  So what kind of parabola will you display on your face today…a negative parabola (a frown?) or a positive parabola (a smile)?

Math or Maths?

I keep seeing the word "maths" on Pinterest.   To me it is confusing and puzzling in addition to being a misspelling or misuse of the word "math".  Where in the world did this word come from, who created it, and why?

Math can be used in the singular form ("math") or in the collective form ("math") without distinction. Math is a field of study which is divided into specific multiple disciplines such as algebra, geometry, etc. which then are again divided into mathematical subgroups.

Likewise, "physics" is single word for a particular field of study, even though it ends in "s". You don't study "a physic"; you study "physics". Just like physics, mathematics is considered singular, but maths? Isn't that like saying deers for deer or sheeps for sheep?

Well, here is the mathematical truth.  Maths is the term commonly used in England, Australia, New Zealand, etc.  It is a shortened form of mathematics.  They pluralize the word, and refer to studying "maths" because mathematics has an "s" on the end.  So the answer to this "maths" question is that it depends on where you live as to what word you use.  Therefore be aware of the geographical differences so you can use the correct form of the word in your writing or speaking.  And when you see the word "maths", don't jump to the conclusion it is a misspelling or a misuse.  Recognize that the writer most likely lives outside of the United States.

Interested in "Math" stuff?  There are 51 resources in my store just for math.  Just go to the category of Math Stuff.