The equation that broke the Internet

The Casio fx-9860 GII SD returns the correct answer to the equation as given. The next row provides the answer that a less sophisticated calculator would return.

6÷2(1+2)=? is the algebra question that many Internet respondents seem to get wrong. If you are in the camp that knows the correct answer is 1, read no further. If you believe the answer is 9, you should probably review this document -- there is only one number that 6 can be divided by to result in 9, and that number is 2/3.

Proof incorrect solution

We substitute x for 2(1+2) from the given equation and backtest the equation with the result of 9:

If

6÷x = 9

then

6÷9 = x

there is a common divisor of 3 so we reduce the fraction...

2÷3 = x

Now we compare the value given in the equation to the value we found (above) for x.

2÷3 != 2(1+2)

Therefore:

6÷2(1+2) != 9

Without fail, 2(1+2) will always equal (2*1 + 2*2), this is the Distributive Law.

Proof of correct solution

Encyclopedia Britannica:

Distributive law, in mathematics, the law relating the operations of multiplication and addition, stated symbolically, a(b + c) = ab + ac; that is, the monomial factor a is distributed, or separately applied, to each term of the binomial factor b + c, resulting in the product ab + ac.

–Encyclopedia Britticana

Therefore, in 6÷2(1+2)=?, we start with the Distributive Law in the Parenthesis step of the Order of Operations.

6÷2(1+2)=?

6÷(2*1 + 2*2)=?

6÷(2 + 4)=?

6÷(6)=?

6÷6=1

Processing rules

In the majority of incorrect proofs the respondent will cite PEMDAS or BODMAS, we all agree that parentheses or brackets are processed first. However, some respondents incorrectly resolve the [P]arentheses or [B]rackets of PEMDAS - BODMAS, for some reason, these solvers believe that they can resolve within the parentheses, but neglect to multiply the adjacent and dependent coefficient.

PEMDAS

P parenthesis

E exponentiation

M multiplication

D division

A addition

S subtraction
–Wolfram PEMDAS

Parentheses

Wolfram Mathworld on parenthetical expressions:

1. Parentheses are used in mathematical expressions to denote modifications to normal order of operations (precedence rules)...

[...]

3. Parentheses are used to enclose the variables of a function in the form f(x), which means that values of the function f are dependent upon the values of x.

–Wolfram Parenthesis

Distributive law

Encyclopedia Britannica on parenthetical expressions:

Distributive law, in mathematics, the law relating the operations of multiplication and addition, stated symbolically, a(b + c) = ab + ac; that is, the monomial factor a is distributed, or separately applied, to each term of the binomial factor b + c, resulting in the product ab + ac.

–Encyclopedia Britticana

Parenthetical expressions

Parenthetical Expression. The parenthesis was described in Chapter 1 as a grouping symbol. When an algebraic expression is enclosed by a parenthesis it is known as a parenthetical expression. When a parenthetical expression is immediately preceded by coefficient, the parenthetical expression is a factor and must be multiplied by the coefficient. This is done in the following manner.
5(a + b) = 5a + 5b

3a(b - c) = 3ab - 3ac
–"Technical Shop Mathematics / Edition 2", by John G. Anderson, ISBN-13:9780831111458, Industrial Press, Inc., 02/28/1983, Page:138

Explicit vs Implicit (implied) multiplication

Implied multiplication has a higher priority than explicit multiplication to allow users to enter expressions, in the same manner as they would be written.
–Implied Multiplication Versus Explicit Multiplication on TI Graphing Calculators

Explicit operations have explicit operators (* ÷ + - ) which are delimiters that divide equations into separate terms. Implied multiplication is notation that informs us that the value of a variable, bracketed function or exponent are connected and not separate terms. Implied multiplication is everywhere, 1x = x and x/1=x. Any number times 1 is that number and any number divided by 1 is that number.

Implied multiplication is also implicit multiplication.

im·plic·it
adjective

1. implied though not plainly expressed.
2. essentially or very closely connected with; always to be found in.
im·plied
adjective

1. suggested but not directly expressed; implicit.
ex·plic·it
adjective

1. stated clearly and in detail, leaving no room for confusion or doubt.
–Oxford

6÷2(1+2) has two EXPLICIT operators, division and addition. However, IMPLIED multiplication tells us to IMPLICITLY multiply, [2(2+1)], in the P step of PEMDAS. In the division step, we must apply division to the (entire) implicitly connected parenthetical expression. We cannot apply division to the coefficient (2) and then not apply division to the factor (2+1).

Incorrect: (because division is not EXPLICITLY applied to the parenthetical expression)

6

2

*

(2+1)

=

3 * (1+2)

Correct: (when EXPLICIT division is applied, it is applied to the entire term.)

6

1

÷

2(1+2)

1

=

2 * (1+2)

6 * 1

Traditional solution:

6

2(1+2)

=

3

1*(1+2)

=

3

= 1

Correctly solving left to right

If we insist on a left to right solution, ignoring both PEMDAS and the Distributive Law, we can do this by expressing the common divisor first. However, division is sustained and must be applied to everything to the right of the explicit division operator.

6÷2(1+2)=?

3÷1(1+2)=?

3÷(1+2)=?

3÷(3)=?

3÷3=1

When we reduce by the common divisor, this does not complete the division operation, it simply reduces the coefficient to one. Division must be applied to both the factor and the coefficient of the parenthetical expression.

The equation is not ambiguous

6÷2(1+2) is not equal to 6÷2*(1+2)

Standing alone, 6 will always equal 6 and 2(1+2) will also always equal 6, so dividing these two terms will always equal 1.

We would not write 2(a+b) as (2(a+b)), nor can we re-write the given equation as 6÷2*(a+b) and maintain the same value. We can however, rewrite the full equation to include an explicit multiplier if we take care to maintain the parenthetical expression with additional bracketing 6÷(2*(a+b)).

Obelus

Granted, the obelus (÷) is an archaic symbol for division, it visually represents a fraction with one dot being the numerator and the other being the denominator. Computers use the forward slash for division because the standard keyboard does not have an obelus key.

For those who claim that the obelus is outdated, why then is it present on virtually every modern calculator?

The calculator problem

The Desmos Scientific Calculator handles our given equation correctly. The error checking appears to be built into the division key.

Entering the equation into a programming language, or low quality calculator, requires the explicit multiplication symbol and outer parenthesis to maintain value of the parenthetical expression.

6÷2(1+2) == 6÷(2*(1+2))

Other calculators, including Google and Wolfram will simply strip the parenthesis and solve a different equation, 6÷2*3. This is because within the programming, the open parenthesis (bracketing) triggers a different function within the programming, an open parenthesis tells the compiler to find the innermost parenthesized term and work outwards. Thus, to get the correct answer from inferior calculators, the input must be formalized with correct bracketing. I.e. 6÷(2*(1+2))

Spreadsheets

Entering the equation as =6/2(1+2) into a cell in a LibreOffice Calc spreadsheet will result in Err:509 (Missing operator), Google Sheets also returns an error (Formula parse error). This forces the user to format the equation using explicit multiplication. To avoid the left to right problem, the compiler must be instructed in advance to prepare for the function with an extra pair of parenthesis =6/(2*(1+2)).