# Mathematics Test for Conceptual Understanding

How well do you understand math concepts? See if you can explain the following 13 statements:

1) “You can’t divide by zero.” Explain why not, (even though, of course, you can multiply by zero.)

2) “Solving problems typically requires finding equivalent statements that simplify the problem” Explain – and in so doing, define the meaning of the = sign.

3) You are told to “invert and multiply” to solve division problems with fractions. But why does it work? Prove it.

4) Place these numbers in order of largest to smallest: .00156, 1/60, .0015, .001, .002

5) “Multiplication is just repeated addition.” Explain why this statement is false, giving examples.

6) A catering company rents out tables for big parties. 8 people can sit around a table. A school is giving a party for parents, siblings, students and teachers. The guest list totals 243. How many tables should the school rent?

7) Most teachers assign final grades by using the mathematical mean (the “average”) to determine them. Give at least 2 reasons why the mean may not be the best measure of achievement by explaining what the mean hides.

8) Construct a mathematical equation that describes the mathematical relationship between feet and yards. HINT: all you need as parts of the equation are F, Y, =, and 3.

9) As you know, PEMDAS is shorthand for the order of operations for evaluating complex expressions (Parentheses, then Exponents, etc.). The order of operations is a convention. X(A + B) = XA + XB is the distributive property. It is a law. What is the difference between a convention and a law, then? Give another example of each.

10) Why were imaginary numbers invented? [EXTRA CREDIT for 12th graders: Why was the calculus invented?]

12) True or false: .99999.... = 1

13) Explain why a negative times a negative is a positive.

I came across this list in Grant Wiggins blog. Note by Calvin Lin
5 years, 9 months ago

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- 5 years, 9 months ago

Lol.

- 5 years, 9 months ago

2) An equal sign is the representation of equivalence. Say that x=y and y=y, the x is a y in disguise.

...that's deep

- 5 years, 9 months ago

3) $\dfrac{a}{b}=a*\dfrac{1}{b}$. Multiply both sides by b

$\dfrac{ab}{b}=a*\dfrac{b}{b}\Rightarrow a=a$

- 5 years, 9 months ago

4). 1/60, 0.002, 0.00156, 0.0015, 0.001

- 5 years, 9 months ago

5) I don't got nutin on 5.... Best I got is that x*x can't be written as addition.

- 5 years, 9 months ago

6) 30, because guarnteed three people won't show.

Jk, 31.

- 5 years, 9 months ago

7) teachers shouldn't take the mean of the grades because there is a lot of effort put into work. Many a time, the student's hard work is overshadowed by one careless error. I'm the sweatiest try hard you'll ever meet. I know how it feels to get a B on something you worked so hard for.

Also, the mean might account for outliers. Say that a student gets a 20/20 on every quiz and one 2/20 (yes, I've seen his happen to many students in my chem class) and one student gets a 20/20 on every quiz but gets two 19/19. If each student gets 1 dropped quiz, then the one who had a lower total before the drop will end up with a higher average.

- 5 years, 9 months ago

8) $F=Y/3; Y=3F; 3=Y/F; 3^3=Y^3/F^3$

- 5 years, 9 months ago

9) idk my vacabilary=0.....punn intended.

- 5 years, 9 months ago

Numbers were invented to quantify things. Because the ancient Romans were too lazy to say that "bill, Sally, joe, Bob, Dan, sean, Dylan... Heather, Sally are present" they just said 15 people are present.

in actuallity, numbers aren't invented. Numbers are to math as letters are to language. One doesn't exist without the other. Numbers were made to prove, without a doubt, why something happens or doesn't.

Yoda, John muradeli, Michael Mendrin, and Brian Charlesworth discuss something similar to this AND MUCH MORE in the comment section here

Calc was also created for these purposes, just for physics stuff.

- 5 years, 9 months ago

12) $0.\overline{9}\neq 1$ because $(0.\overline{9})^\infty=0$ while $1^\infty=1$ if two values are equal, then putting those values to any power should keep them equal.

- 5 years, 9 months ago

13) I tried to argue this, but it became a cyclic arguement. So all that I can say is that if u tell some one "I do not not want this" its like saying that "I want this".

(I know in mathematical logic this is flawed because it technically doesn't mean u like it, it just means that u don't not want it.)

- 5 years, 9 months ago

When you figure this out (as in a proper formal proof), you should add it to Trivial Inequality.

Hint: What is a definition of positive number that doesn't involve 0?

Staff - 5 years, 9 months ago

Maybe Multiplying by 0 or by negative numbers

- 5 years, 9 months ago

multiplying by 0 is like adding it 0 times and multiplying by a negative is like adding a negative x amount of times.

- 5 years, 9 months ago

I'm trying to actually solve this. I saw the answer for #1. It says that if you consider 1/0 to infinity, 1/0 * 0 = 1, if you cancel 0 or 1/0 * 0 = infinity * 0 = 0. This leads to ambiguity of saying (any number)/0 is infinity.

- 5 years, 9 months ago

In #2 No idea.

- 5 years, 9 months ago

In #3 $\frac{x}{\frac{a}{b}} = x * \frac{b}{a}$ $\frac{x}{\frac{a}{b}} * \frac{b}{b} = \frac{xb}{a}$ $\frac{xb}{\frac{ab}{b}} = \frac{xb}{a}$ $\frac{xb}{a} = \frac{xb}{a}$

- 5 years, 9 months ago

I think #2 is a bit too sweeping of a statement. It sort of reminds me how Richard Feynman [jokingly] proposed a GUT, which is to round up and state all equations of physics in the form $X=0$ and then multiply them all to get $\prod { X } =0$. Yes, it'd be a GUT, but it wouldn't be very illuminating, wouldn't simplify anything, and wouldn't help us learn anything new.

The equals sign, $=$, is actually used in a variety of context, as for example, in an identity, where both sides are always the same, or in an assignment, where a variable is given a specific value, or in an definition, as for example a Log function is defined to be the inverse of an exponential, etc. Not all equals are equal. One who programs computers would be familiar with these kinds of issues. To say that "solving problems typically requires finding equivalent statements that simplify the problem", suggesting that problem solving is a chain of equal quantities from unsolved to simple solved is just grossly oversimplifying the practice of doing mathematics.

- 5 years, 9 months ago

10)

to find the root of

$x^{2} + 1 = 0$

- 5 years, 9 months ago

actually it was created by a teacher to prevent his students from taking root of -1 x root of -1 is equal to 1 that is how 'i' was created.

- 5 years, 9 months ago

13)

x any be real numbers

$- x . - y + ( -x . y) = -x( - y + y) =0$

thus negative . negative is equal to positive

sir is this right?

- 5 years, 9 months ago

12) let us suppose that there are n number of 9

we can write it in this way

$\frac{9999.....}{10^{n}}$

$= \frac{10^{n} - 1}{10^{n}}$

$= 1 - \frac{1}{10^{n}}$

thus as n will go on increase $\frac{1}{10^{n}}$ go on decrease (tends to 0)

thus $= 1$

- 5 years, 9 months ago

(1)Proving that you cannot divide by 0: First I will prove that you cannot divide a $\textbf{non-zero}$ number by 0.Assume that proving by 0 is possible.Let $a$ be any non-zero real number.Then $\frac{a}{0}$ results in a number $b$ which can be written as: $\frac{a}{0}=b\\a=b\times 0$ Anything times 0 is 0,so $b$ times 0=0.But then the equation becomes $a=0$.But we have already fixed that a is non-zero.Therefore,we have arrived at a contradiction.Therefore we cannot divide non-zero numbers by 0. Second part of proof:Proving that 0/0 is undefined: Let 0/0 be equal to $c$ Then: $\frac{0}{0}=c\\0=c\times 0$ But LOOK:$c$ can be any number;it can be $0,1,\frac{1}{2},\pi\;etc$ $\textbf{ANY NUMBER}$.But a division can only have 1 answer.So it is proved that $\large{\textbf{YOU CANNOT DIVIDE BY 0}}$

(4)1/60,0.00156,0.0015,0.002,0.001

(5)$\sqrt{2}\times\sqrt{3}$,which cannot be expressed as repeated addition.In general $\sqrt{a}\times\sqrt{b}=\sqrt{ab}$ cannot be expressed as repeated addition if both $a$ and $b$ are not perfect squares. $\textbf{Explanation:}$You can express $a_1\times a_2 \times a_3\times\dotsm a_n$ as repeated addition if any one of the numbers from $a_1$ to $a_n$ is an integer.But this is not necessarily true.

(6)You cannot order 30 tables because 3 people will still be left standing so you have to order 31 tables.

(7)For example,if a teacher assigns a final grade by using the mean of a students with marks:19/20,18/20,20/20,19.5/20 and one really bad 10/20 (Which does happen) the student's final grade will be really low because of that 10/20.In such cases,it is better to use other types of averages.

(8)$F=\frac{Y}{3}$

(10)To find solutions to those equation which had no real solutions.

(12)True $\textbf{Proof:}$Let: $0.999\dots=x\\9.9999\dots=10x\\10x-x=9.999\dots-0.999\dots\\9x=9\\x=\frac{9}{9}=\boxed{1}$ (13)$0=(-1)\times0\\=(-1)\times[1+(-1)]\\=(-1)(1)+(-1)(-1)\\=-1+(-1)(-1)=0\\-1+(-1)(-1)=0+1\\\boxed{(-1)(-1)=1}$ So $(-x)\times(-y)=(-1)\times(-1)\times x\times y=1\times xy=xy$

- 5 years, 7 months ago

for number 12: its true using geometric infinite series.

- 5 years, 9 months ago

Multiplication is repeated addition only for integers, we cannot define it for real numbers where we cannot express it as such,., in fact if multiplication was really just repeated addition,,

then x^2=x+x+x+x......x (x times for all real no.) and on differentiating, we get 2x=1+1+1+1+... (x times) = x in which case we get 2x=x for all real number which is false

- 5 years, 9 months ago

I disagree with #1.

Depends on how you define "dividing," you can actually get a "defined" result.

In the Riemann Sphere, s

For this set of arithmetic, one may yield, for any nonzero $a$, $\frac{a}{0}=$ complex infinity.

- 5 years, 9 months ago

1)

Using binary operation on set of complex numbers , when we divide any complex number by zero it does not belongs to its set

Using binary operation on set of complex numbers , when we multiply any complex number by zero it belongs to its set

therefore we can multiply any number with zero but can't divide

IS this right?

- 5 years, 9 months ago

The first statement says that "You can't divide by zero." You can simply say

$\frac{x}{0}$

is impossible because...

$y \times 0 = x$ ,

in which where $x \neq 0$. No such real number can stand for $x$.

- 5 years, 9 months ago

In which somehow, any number, such as $y$, multiplied to $0$ is ALWAYS equal to $0$.

- 5 years, 9 months ago

help me learn here:

@no.1- zero implies "infinite space where infinite number possible can fit." You can't possibly divide something or anything with an infinite space where infinite number possible could exist.

Am I making sense in my statement?

- 5 years, 8 months ago

1) we cannot divide by zero because the result is undefined where as if we multiply by zero then the result itself is zero.

- 5 years, 8 months ago

6) No. Of tables required= 31

- 5 years, 8 months ago

12). True

- 5 years, 8 months ago

12) 0.999.... = 1 It is only a matter of notation. No mystery at all.

- 5 years, 9 months ago

# 1 we cannot divide by 0 but multiply because in division we not only have 1 but 2 conditions. in multiply it creates no problem because the answer is always 0 and we don;t need to do discussion over that , but in division we have to find the nearest multiple of any positive or negative no. for eg. - 6 . but even if we multiply 0 *10000000000 we know we would never reach to 6. and hence it just results to be ∞

- 5 years, 9 months ago

how did you increase the font size?

- 5 years, 9 months ago