Squares and Square Roots | IT

Question 2

Why does the new dotted square have double the area of the original square?

In many of the constructions in the Śulba-Sūtra, it is desirable to construct, where needed, what Baudhāyana calls ‘east-west’ and ‘north-south’ lines, i.e., horizontal and vertical lines that are perpendicular to each other. Can you draw some horizontal and vertical lines to see why the new square has double the area of the original square? You could draw some horizontal and vertical lines as shown on the right.

Solution

Understand the Question

Let the side length of the original square be $s$ , so its area is $s^2$ .
By constructing horizontal and vertical ('east-west' and 'north-south') grid lines, we can enclose both squares inside a larger square of side length $2s$ and area $4s^2$ .
The region between the large enclosing square and the new tilted/dotted square consists of $4$ congruent corner right-angled triangles, each of base $s$ and height $s$ .
Subtracting the total area of these $4$ triangles from the large square gives the area of the new dotted square as $2s^2$ , which is exactly twice the original area.

Step 1 · Find Area of the Original Square

Let the side length of the original solid square be $s$ . Diagram 1

$\text{Area of original square} = s \times s = s^2$

Step 2 · Find Area of the Enclosing Square

Draw horizontal and vertical lines to form a large enclosing square around both squares with vertices at $(0,0)$ , $(2s,0)$ , $(2s,2s)$ , and $(0,2s)$ . Diagram 2

\begin{aligned} \text{Side length of enclosing square} &= 2s \\[0.6em] \text{Area of enclosing square} &= (2s) \times (2s) \\[0.6em] &= 4s^2 \end{aligned}

Step 3 · Calculate Area of the Four Corner Triangles

The region outside the new dotted square inside the large square forms $4$ identical right-angled triangles, each with legs of length $s$ .

\begin{aligned} \text{Area of one corner triangle} &= \dfrac{1}{2} \times s \times s \\[0.6em] &= \dfrac{s^2}{2} \end{aligned}

\begin{aligned} \text{Total area of four triangles} &= 4 \times \dfrac{s^2}{2} \\[0.6em] &= 2s^2 \end{aligned}

Step 4 · Find Area of the New Dotted Square

Subtract the area of the four corner triangles from the area of the enclosing square. Diagram 4

\begin{aligned} \text{Area of new dotted square} &= \text{Area of enclosing square} - \text{Total area of four triangles} \\[0.6em] &= 4s^2 - 2s^2 \\[0.6em] &= 2s^2 \end{aligned}

Step 5 · Compare the Areas

Comparing the areas of both squares:

$\text{Area of new dotted square} = 2s^2 = 2 \times (s^2) = 2 \times \text{Area of original square}$

Answer

The new dotted square has an area of $2s^2$ , which is exactly double the area of the original square ( $s^2$ ).

Common Mistakes

Assuming side length doubles: Doubling the side length of a square quadruples its area ( $4s^2$ ), whereas here the side length is $\sqrt{2}s$ , doubling the area to $2s^2$ .
Missing Corner Triangles: Forgetting to subtract all $4$ corner triangles from the total $4s^2$ area of the enclosing square.

More questions in IT

How can one construct a square having double the area of a given square?

Why does the new dotted square have double the area of the original square?

Why should the extension of the vertical and horizontal sides of the original square pass through the vertices of the dotted square?

[Hint: From the diagonal property of a square, the line that bisects an angle passes through the opposite vertex. Argue why the vertical and horizontal sides of the original square bisect the two angles of the dotted square.**]

Context: So, the new square has double the area of the original square, because the original square is made up of two small triangles, while the new square is made up of four small triangles.

Q. Moreover, all these small triangles are congruent to each other. Can you explain why?

Now suppose we are given a square, and we want to construct a square whose area is half that of the original square. How would you do it?

Why is the smaller inside square half the area of the larger square?

Again, adding some east-west and north-south lines can explain it:

Why is $PQRS$ a square? Why is its area half that of the original paper?

Explain by connecting $QS$ and $PR$ , finding the different angles formed, and then using tringle congruence.

Find the hypotenuse of this isosceles right triangle.

What is the value of $\sqrt{2}$ ?

Q10

Is $\sqrt{2}$ less than or greater than 1?

Q11

Is $\sqrt{2}$ less than or greater than 2?

Q12

Can we find closer bounds for $\sqrt{2}$ ?

Q13

Will we ever get a number with a terminating decimal representation whose square is 2?

If there is such a terminating decimal starting with $1.414\dots$ whose square is 2, then it must have a non-zero last digit. If this is the case, then the decimal representation of its square will also have a non-zero last digit after the decimal point. For example, if $\sqrt{2}$ is of the form $1.414\dots4$ , then its square will be of the form—

Q14

Use this formula to check your answers in the Figure it Out on page 39.

Q15

What if we wish to combine two squares of 'different' sizes to make a large square whose area is the sum of the two smaller squares?

Q16

Why does Baudhāyana’s method work?

Q17

Can you see why the method works in the case where the two squares are the same size? Does it agree with the method we used earlier to combine two same sized squares into a bigger square?

Q19

Explain why all the angles of this new 4-sided figure are right angles and so it is a square.

Q20

List down all the Baudhāyana triples with numbers less than or equal to 20.

Q21

Is there an unending sequence of Baudhāyana triples?

Q22

Is $(30, 40, 50)$ a Baudhāyana triple?

Is $(300, 400, 500)$ a Baudhāyana triple?

Q23

Context: The list of Baudhāyana triples having numbers less than or equal to 20 contains the following triples — $(3, 4, 5)$ , $(6, 8, 10)$ , $(9, 12, 15)$ , $(12, 16, 20)$ .

Q. Do you see any pattern among them?

Q24

Context: All these triples can be obtained by multiplying each term of $(3, 4, 5)$ by a certain positive integer.

Q. Can we form a conjecture on Baudhāyana triples based on this observation?

Q25

Context: Conjecture: $(3k, 4k, 5k)$ is a Baudhāyana triple, where $k$ is any positive integer.

Q. Is this true?

Q26

Is $(5, 12, 13)$ a primitive Baudhayana triple? What are the other primitive Baudhayana triples with numbers less than or equal to $20$ ?

Q27

Generate 5 scaled versions of each of these primitive triples. Are these scaled versions primitive?

Q28

If $(a, b, c)$ is non-primitive, and the integers have $f$ — greater than 1 — as a common factor, then is $\left(\dfrac{a}{f}, \dfrac{b}{f}, \dfrac{c}{f}\right)$ a Baudhayana triple? Check this statement for $(9, 12, 15)$ . Justify this statement.