Checking – Catching Mistakes Before They Catch You

Everybody makes mistakes from time to time. It’s one of the ways we learn. It’s also something that distinguishes us from computers. When I was a very junior programmer, one of the team leaders in the office said to me, “The machine is doing exactly what you told it to do, lad. It’s just not doing what you thought you told it to do.” I repeat, everyone makes mistakes…

This week’s post is about ways of checking your work. Some of them give you an absolute guarantee that you are right. Some of them merely show that an error exists. They’re worth doing, either way.

Mental Arithmetic

Use these when you have to work it out by hand, or as checks against typos when using a calculator.

Last digit checks can highlight errors in multiplication. The most basic comes from the simple fact that you only get an odd answer when you multiply odd numbers. However big the multiplication, a single even number makes the answer even. One factor of two is all it takes… (sounds like an advertising slogan). So if you multiply a string of odd numbers and get an even answer (or vice versa), something has gone wrong.

A more sophisticated last digit multiplication check is the units make the units. For example: 27346255346277347 x 38482942904939 must have an answer ending in a 3. This is because 9 x 7 = 63 and although the 60 gets mixed in with the other tens, nothing else can affect the 3 in the units column. Try it with smaller numbers and you will see what I mean.

Ballpark estimates are always a good idea. See if your answer seems ‘about right’ by rounding to simple whole numbers. In particular, take π as 3, and gravity as 10 ms-2. In a recent lesson we looked at a question involving a stone being thrown vertically downwards from the top of a tower with initial speed 4ms-1, and taking 2 seconds to reach the ground. The problem was to find the height of the tower.

This is easy enough using s = ut + ½at2 = (4 x 2) + (½ x 9.8 x 22 ) = 27.6 m

But if you miss out the decimal point in gravity (and use 98) you get 276m as the answer. This is a tall tower, but not impossibly so.

The ballpark check 4×2 = 8, plus ½ x 10 x 4 = 20, giving 28 will highlight the error faster than you can write it down (or the stone takes to fall).

Counting minuses in multiplication (or division) is a simple way of making sure you haven’t got the sign wrong. If you have an even number of minuses, the answer is positive. If you have an odd number of minuses, the answer is negative. Easy!

Counting decimal places is worthwhile whenever you have to multiply decimals. The rule is that you always end up with the same number of d.p. that you start with. This may include trailing zeros. Here are some examples:

0.5 x 0.5 = 0.25 2 dp on either side

0.24 x 0.45 = 0.1080 4 dp on either side.           (Calculators just show 0.108)

0.02 x 0.0004 = 0.000008 5 dp on either side

In the last example, counting the d.p. on the left hand side is a quick way of determining that we need five leading zeros to bring the total d.p. to 6.

Calculator checking

Comparing answers. If you are working from a text book and checking your answers against the ones in the back, you sometimes get a situation where the forms of the answers differs.

For example you might have 0.805612004, and the book has 3e-5 + π/4.

A quick way of checking is to type in the book version, followed by divide and ANS (the button that recalls the answer you just had). If you are right, the new answer will be 1. You could make the same test by subtracting (answer = 0 if correct) but I always use division. No particular reason.

Simplifying algebra From time to time you may have to ‘simplify’ horrible pieces of algebra. For example:


To check this, pick a couple of easy values for x and y (Don’t choose anything that will lead you to divide by zero!). So with x = 1 and y = 2, we have:

Selection_010which gives Selection_011

It only takes a few seconds to type this into your calculator using the fractions button. You could even do it by comparing answers by entering:

Selection_012 (there’s now a minus in the middle)

This gives an answer of zero straight away.

Checking solutions to equations is easy with a calculator if you use the memory to hold variables. (I am assuming you have a typical school scientific calculator such as a Casio fx-83GT).

Step 1 Assign the value you want to check to the variable X by entering the value then pressing shift, RCL, ). For example:

7 Shift RCL ) sets X to 7.

You don’t need to press =. The display shows 7X at the top. Any previous X value is overwritten.

Step 2 Enter your expression, using the Alpha button whenever you want X.

Alpha ) x2 – 2 Alpha ) + 3 gives you x2 + 2x + 3

Step 3 Now press =. The calculator works out 72 – (2×7) + 3 to give 38.

Step 4 Now press AC, then enter the next value you want to test. For example:

5 Shift RCL ) sets X to 5.

Step 5 Press the up arrow. (This scrolls you back through previous actions). The expression x2 + 2x + 3 will reappear. Now press = again, to get the answer with X = 5.

You can do this as many times as you want, and if you are dealing with awkward decimals or surds it is quicker than typing them into the expression directly. It’s probably overkill at GCSE but can be a great help at A level where you often have equations with multiple solutions. (Tip If you aren’t sure of the current value of X, press Alpha ) = to display it.)

These are just some useful checking tricks. Maybe you have your own favourites you would like to share.

Calculators can bite!

Calculators have become as ubiquitous as pencils, and you have to be at least as old as I am to remember a time when they were an expensive luxury that you weren’t allowed to use in school – like mobile phones were only a few years ago! However, also like phones, most people don’t really know half the things their calculator does – and even when you think you do, they can still catch you out, as I discovered yesterday in a lesson. (Toby, if you’re out there this one’s for you!)

So here are a few things you might want to remember. They refer to the standard Casio fx-83 machines that are commonest in schools these days. (Mine is an fx-83GT PLUS).

1. The manual

Don’t throw it away with the packaging! It’s not the easiest thing to read, but it is useful, especially when your best friend has switched your calculator into a mode you don’t understand and can’t get it back.

If you have thrown it away, or the dog ate it as a side dish to your homework, all is not lost. You can download Casio manuals from here:

2. Modes

Most of the time, you will want mode 1 (COMP) for ordinary computation. Press the MODE button, then choose option 1.

The TABLE mode (mode 3) is useful if you have to work out lots of values to plot a graph of an equation. f(x) = means the same as y = for this. Enter the equation using the ALPHA button followed by the right bracket button to get an x when you need one. Then specify the start and end values you want, and the step value (that’s the gap between your x values), press enter and you will have a table of x and y values all ready to plot. And remember to put the calculator back into COMP mode when you’re done.

3. Setup

Enter the setup menu by pressing SHIFT then MODE. Options 1 and 2 let you choose how the calculator displays fractions, surds and so on. You will probably prefer option 1 (MthIO), followed by output option 1 (MathO). These let you enter fractions the way they look on paper, and display results as fractions, surds and multiples of Pi whenever possible. To get the decimal equivalent, press the SD button.

You shouldn’t need the other setup options for GCSE, unless somebody (perhaps a friend in the sixth form) has changed the way your calculator handles angles. You want option 3 for degrees. A letter D appears at the top of the display when this has been selected.

4. Recurring decimals

If you enter 1/7 then press = , the display will show 0.142857 with dots over the 1 and the 7 to show that these digits recur. (Pressing = again gives as many digits as the display will hold, with the last digit rounded. A third press gets you back to the fraction).

You can also enter a recurring decimal by hand, by pressing SHIFT followed by the x2 button when you want to enter the repeated digits. You get a box with the dot over it, and type the numbers you want into it. Typing 0 . SHIFT x2 142857 = will give an answer of 1/7. This is great for checking those ‘recurring decimal to fraction’ problems, but won’t get you any marks in the exam, as you have to show your working.

Very large recurring sequences may not show the right hand dot straight away. For example 1/17 displays 0.0588235294117 with a dot over the first decimal place and an arrow pointing right after the 7. Pressing the right arrow key three times displays an extra 647 with the right hand dot over the final digit.

5. Factors

You can express an integer as a product of prime factors by pressing SHIFT followed by the ‘degrees, minutes, seconds’ button underneath the square root button. For example, 47952 is expressed as 24 x 34 x 37. For some reason, the calculator only factorises answers, so you need to type in the number, press = and then factorise it.

(In passing, it mystifies me that every calculator I have owned in the last 20 years has the ‘degrees, minutes, seconds’ button, which I have never used (not once!) and yet requires me to press SHIFT every time I want the Pi button. Sharp make calculators with a top level Pi button, but for some reason Casio choose not to…)

6. Squaring negative numbers

Calculators do BIDMAS! That is, when you enter something they look at brackets first, then indices, then division, multiplication, addition and subtraction.

This means that when you enter -3 then press x2 = you get an answer of -9.

You did know that’s wrong, didn’t you? (A minus times a minus makes a plus?)

It only does this when you actually type in the negative number. (It ‘knows’ the sign of the answer to something like 4 – 7 and will square it correctly.) So to make sure you aren’t embarrassed by a negative square:

a) Type in the number, press =, then square it.

b) Put brackets round the number, then square.

c) Don’t put the minus sign in at all!! After all, you know the answer should be

positive. Save keystrokes, save energy.

7. Mixed fractions

The best way to enter these is to press the shift button followed by the fractions button (in MthIO mode), then type the numbers into the boxes.

e.g for 2 ½ press shift, fraction, 2, right arrow, 1, down arrow, 2

This gives you the correct decimal value of 2.5 when you press = and SD.

The way NOT to do it is to type the fraction, then backspace and enter the whole number to the left of it.

e.g fraction, 1, down arrow, 2

then either right arrow twice or left arrow 4 times to get the cursor on the left side,

then 2 for the whole number

If you now press =, the answer is 1.

The calculator has interpreted what you did as 2 x ½ !!

I have no idea why this happens, but we had a fun time in yesterday’s lesson figuring out exactly what was going on….

Don’t do it!