# Tag Archives: simple

## GEOSTRUCT, a program for investigative geometry

I have been developing this computer software / program / application for some years now, and it is now accessible as a web page, to run in your browser.

It provides basic geometric construction facilities, with lines, points and circles, from which endless possibilities follow.

Just try it out, it’s free.

Click on this or copy and paste for later : www.mathcomesalive.com/geostruct/geostructforbrowser1.html

.Here are some of the basic features, and examples of more advanced constructions, almost all based on straightedge and compass, from “make line pass through a point” to “intersection of two circles”, and dynamic constructions with rolling and rotating circles.

Two lines, with points placed on them

Three random lines with two points of intersection generated

Five free points, three generated circles and a center point

Three free points, connected as point pairs, medians generated

Two free circles and three free points, point pairs and centers generated

GIF showing points of intersection of a line with a circle

Construction for locus of hypocycloid

GIF showing a dilation (stretch) in the horizontal direction

Piston and flywheel

Construction for circle touching two circles

Construction for the locus of a parabola, focus-directrix definition.

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Filed under education, geometry, math, operations, teaching

## Subtraction and the “standard” algorithm

CCSSM talks about “the standard algorithm” but doesn’t define it – Oh, how naughty, done on purpose I suspect, since there are varieties even of the  “American Standard Algorithm”. Besides, if it is not defined it cannot be tested (one hopes!). I checked some internet teaching stuff on it, and as presented it won’t work on for example 403 – 227 without modification.

Anyway, I was thinking about subtraction the other day (really, have you nothing better to think about?), and concluded that subtraction is easiest if the first number ends in all 9’s or the second number ends in all 0’s. So, fix it then, I thought, change the problem, and here are the results

.

I am quite sure that some of you can extract the general rule in each case, and see that it works the same in all positions.

“If I understand subtraction, and can explain the ideas to another, and I learn the standard algorithm and how to apply it, and I have faith in it based on experience, WHY THE HELL DO I HAVE TO BE ABLE TO EXPLAIN IT?”

I guess this post counts as a rant!

Filed under arithmetic, math, operations, subtraction, teaching

## Let’s actually use the number line

Let’s have a number line. We can count up or down by moving to the right or the left, or actually up or down if the number line is drawn vertically.
But what else ?????

The following pictures show how points on the number line which represent fractions can be found exactly by simple geometrical construction, and then how results of multiplication and division of fractions can be found exactly as points on the number line (sorry, the numerical values are however not found).

This arose from a statement in the CCSS math document that the fraction 1/6 could be represented by a point one sixth of the way from zero to one, BUT NOWHERE DOES IT SAY HOW TO FIND THAT POINT.

Filed under arithmetic, fractions, geometry

## Another way to do subtraction

No more borrowing and paying back, or the new alternative

Use addition and then simple subtraction, naturally there are several steps

Example     234 – 187

234

-187  (should be lined up!)

Now 7 is greater than 4,  so we add 3 to both numbers, giving a zero in the units position of the second number

234 -187 = 237 – 190   (must be able to add 3 to 187 !!!)

moving to the 10’s position, and very formally, 90 is greater than 37 so add 10 to both numbers, giving

237 – 187 = 247 – 200, which is now simple, with result  47

Of course, if the lower digit is less than the one above just subtract it from each number

234 – 117 = 237 – 120 = 214 – 100 = 114

It is best to keep the “one above the other” layout, but I have fought with the editor enough today.

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Filed under arithmetic