Question 19

Observe the way a compass is made. What can one draw with the compass? Explore!

Take a point on the circle. What will be its distance from $P$—equal to $4\text{ cm}$, less than $4\text{ cm}$ or greater than $4\text{ cm}$? Similarly, what will be the distance between $P$ and another point on the circle?

3. Eyes

How do you draw these eyes with a compass?

What shapes are these? Yes, these are our familiar squares and rectangles. But what makes them squares and rectangles?

Which of the following is not a name for this square?

1. $PQSR$
2. $SPQR$
3. $RSPQ$
4. $QRSP$

Can you see why $\text{PS}$ should be $6\text{ cm}$ long?

How long is the side $RS$ and what are the measures of $\angle R$ and $\angle S$?

Draw a rectangle with sides of length $4\text{ cm}$ and $6\text{ cm}$. After drawing, check if it satisfies both the rectangle properties.

Draw a rectangle of sides $2\text{ cm}$ and $10\text{ cm}$. After drawing, check if it satisfies both the rectangle properties.

Is it possible to construct a 4-sided figure in which—

• all the angles are equal to $90^\circ$ but
• opposite sides are not equal?

In a rectangle $ABCD$ with $AB = 7 \text{ cm}$ and $BC = 4 \text{ cm}$, $X$ is a point that can be moved anywhere along the side $AD$, and $Y$ is a point that can be moved anywhere along the side $BC$.

Q. At which positions will the points $X$ and $Y$ be at their closest? When do you think they will be the farthest? What does your intuition say? Discuss with your classmates.

In a rectangle $ABCD$ with $AB = 7\text{ cm}$ and $BC = 4\text{ cm}$, $X$ is a point that can be moved anywhere along the side $AD$, and $Y$ is a point that can be moved anywhere along the side $BC$.

Q. Verify your guesses by placing the points $X$ and $Y$ on the sides and measure how near or far they are. The distance between $X$ and $Y$ can be obtained by measuring the length of the line $XY$.

(i) How does the minimum distance between the points $X$ and $Y$ compare to the length of $AB$? (ii) Change the positions of $X$ and $Y$ to check if there are other positions where they are at their nearest or farthest. How will you keep track of the lengths $XY$ for different positions of $X$ and $Y$?

In a rectangle $ABCD$ with $AB = 7\text{ cm}$ and $BC = 4\text{ cm}$, $X$ is a point that can be moved anywhere along the side $AD$, and $Y$ is a point that can be moved anywhere along the side $BC$.

Q. Suppose here are some of the positions of $X$ and $Y$ that you have considered. Find the length of $XY$ for each case shown in the list below:

Q. Is there a shorthand way of writing it down? In all the sentences, only the position of $X$, $Y$ and the length $XY$ changes. So we could write this as shown in the table below:

Have you checked what happens to the length $XY$ when $X$ and $Y$ are placed at the same distance away from $A$ and $B$, respectively? For example, as in the cases like these:

and so on.

In each of these cases, observe

1. how the length $XY$ compares to that of $AB$ and
2. the shape of the 4-sided figure $ABYX$.

How does the farthest distance between $X$ and $Y$ compare with the length of $AC$? $BD$?

Explore

What about constructing a rectangle that can be divided into two identical squares? Can you try it?

It is wise to first plan and then construct. But how do we plan? Can you think of a way?

To draw the rectangle $ACDF$, one could assign any length to $AF$. For example, if we assign $AF = 4 \text{ cm}$, then what must the length of $AC$ be?

With this idea, try constructing a rectangle that can be divided into three identical squares.

Give the lengths of the sides of a rectangle that cannot be divided into—

• two identical squares;
• three identical squares.

Construct

1. A Square within a Rectangle

Construct a rectangle of sides 8 cm and 4 cm. How will you construct a square inside, as shown in the figure, such that the centre of the square is the same as the centre of the rectangle?

Falling Squares

Construct the figure shown below, where each is a square of side $4\text{ cm}$. Make sure that the squares are aligned the way they are shown.

Now, try this: Construct the figure with squares of side $3\text{ cm}$, $5\text{ cm}$, and $7\text{ cm}$ as shown.

Shadings

Construct this. Choose measurements of your choice. Note that the larger 4-sided figure is a square and so are the smaller ones.

4. Square with a Hole

Observe that the circular hole is the same as the centre of the square.

Hint: Think where the centre of the circle should be.

5. Square with more Holes

6. Square with Curves

This is a square with $8\text{ cm}$ sidelengths.

Hint: Think where the tip of the compass can be placed to get all the 4 arcs to bulge uniformly from each of the sides. Try it out!

8.5 Exploring Diagonals of Rectangles and Squares

Consider a rectangle $PQRS$. Join $PR$ and $QS$. These two lines are called the diagonals of the rectangle.

Compare the lengths of the diagonals. First predict the answer. Then construct a rectangle marking the points as shown and measure the diagonals.

Observe that a diagonal divides each of the pair of opposite angles into two smaller angles. In the figure, the diagonal $PR$ divides angle $R$ into two smaller angles which we simply call $g$ and $h$. The diagonal also divides angle $P$ into $c$ and $d$. Are $g$ and $h$ equal? Are $c$ and $d$ equal?

First predict the answers, and then measure the angles. What do you observe? Identify pairs of angles that are equal.

Check if $ABCD$ is indeed a rectangle satisfying properties $R1$ and $R2$.

Construct

1. Construct a rectangle in which one of the diagonals divides the opposite angles into $50^\circ$ and $40^\circ$.

Construct

2. Construct a rectangle in which one of the diagonals divides the opposite angles into $45^\circ$ and $45^\circ$. What do you observe about the sides?

Construct

3. Construct a rectangle one of whose sides is $4\text{ cm}$ and the diagonal is of length $8\text{ cm}$.

Construct

4. Construct a rectangle one of whose sides is $3\text{ cm}$ and the diagonal is of length $7\text{ cm}$.

Was it necessary to draw two full circles to get the point $A$? We only needed part of both the circles.

Having obtained point A, what remains is the construction of the remaining arc. How do we do it?

Can we use the fact that A is of distance $5\text{ cm}$ from both B and C?

Construct a bigger house in which all the sides are of length 7 cm.

Try to recreate 'A Person', 'Wavy Wave', and 'Eyes' from the section 'Artwork', using ideas involved in the 'House' construction.

Is there a 4-sided figure in which all the sides are equal in length but is not a square? If such a figure exists, can you construct it?

B) (From Construct above (page no. 211).)

For the purpose of construction, let us take the side lengths to be of $5\text{ cm}$. Consider this figure.

We need to identify only one more point to make this a 4-sided figure. That point, let us call it $D$, should be $5\text{ cm}$ from both $B$ and $C$. How can such a point be found? Can any of the ideas used in the 'House' problem be used here?