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Section 3.1 Pythagorean relationship

Definition 3.1.1.

The Pythagorean relationship is a fundamental principle in geometry that describes the relationship between the sides of a right triangle. It states that the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. This relationship is expressed by the equation \(a^2 + b^2 = c^2\text{,}\) where \(c\) is the length of the hypotenuse, and \(a\) and \(b\) are the lengths of the other two sides.

Subsection 3.1.1 Pythagorean relationship

  • A triangle that contains a right angle (an angle that measures \(90^\circ\text{,}\) symbolized by a small square "β–‘") is called a right triangle.
  • The longest side of the right triangle (the side opposite the \(90^\circ\) angle) is called the hypotenuse and the other two (shorter) sides are called the legs of the triangle.
  • The legs of a right triangle are commonly labelled \(a\) and \(b\) while the hypotenuse is labeled \(c\text{.}\)
  • The Pythagorean relationship is a special rule about right-angled triangles. It helps us find the length of one side if we know the other two sides.
    We use the Pythagorean relationship in many real-life situations, such as:
    \begin{equation*} {\color{green} \text{Carpentry}}\text{: To design classrooms, furniture and divide land into plots.} \end{equation*}
    \begin{equation*} {\color{green}\text{ Building and Room Design}}\text{: For example, when building classrooms or arranging furniture.} \end{equation*}
    \begin{equation*} {\color{green} \text{Sports}}\text{: In games like basketball, soccer, or football, it helps measure distances, like how far a player runs or the shortest path to the goal.} \end{equation*}
  • A long time ago, about 2000 years back, people discovered something amazing about right-angled triangles. They found that:
    \({\color{green} \text{If you make squares on each side of a right-angled triangle, the area of the two smaller squares adds up to the area of the largest square.}}\)
    In other words, the two small squares together are exactly the same size as the big square.
    The ubove description for about 2000 years
    This special rule is called the Pythagorean Theorem, and we write it as:
    \begin{equation*} a^2+b^2=c^2 \end{equation*}
    Where,
    \(a \) and \(b \) are the shorter sides of the triangle.
    \(c \) is the longest side (called the hypotenuse).

Activity 3.1.1.

Work in Groups.
Read the story below.
Cherotich and Onyango are Grade 7 learners. During the August holiday, they visited their grandmother, who lives nearby a village. Their grandmother is a grocer. Cherotich and Onyango help there grandmother to fetch fruits from a tree for selling. Cherotich used a \(10\,m\) ladder to climb the fruit tree. The ladder had its foot \(6\,m\) from the bottom of the tree and leaned on the tree at the height of \(8\,m\text{.}\)
The tree
\({\color{blue}\textit{Learning point}}\)
  1. Draw the figure that has been formed between the tree, ladder and the ground?
  2. How many sides does it have?
  3. Which is the longest side of the figure?
  4. Identify the two shorter sides of the figure.
  5. Name the side of the figure. You may use digital devices to research on the internet.
  6. Share your findings with other leaners in class.
Solution.
Here is the correct Learning point.
  • The figure formed between the tree, ladder and the ground is a right-angled tringle.
  • A right-angle triangle has three sides: the base, the perpendicular height and the hypotenuse.
  • The side that is \(8 m\) long is the perpendicular height.
  • The side that is \(6 m\) long is the base.
  • The side that is \(10 m\) long is the hypotenuse.
  • The longest side in a right-angled triangle is the hypotenuse.
  • The two shorter sides of this figure are the base and the perpendicular height.
\({\color{blue}\text{The pythagorean relationship.}}\)
  • It state that, the sum of the squares of the two shorter sides is equal to the squre of the longer side (hypotenuse).
  • From the figure alongside, side \(a\) is the \({\color{blue}\text{base}}\text{,}\) side b is the \({\color{blue}\text{height}}\) and side c is the \({\color{blue}\text{hypotenuse}}\text{.}\)Therefore the relationship is, \(a^2+b^2=c^2\text{.}\)
    The relationship can be changed depending on the question asked on the length to calculate as shown below.
  • \begin{equation*} c^2=a^2+b^2 \end{equation*}
    \begin{align*} b^2= \amp c^2-a^2\\ a^2= \amp c^2-b^2 \end{align*}

Example 3.1.2.

Find the length of the side marked y in the right-angled triangle below.
Hint.
To find the length of any side of the right-angled triangle, we use the pythagorean relationship which state that: \(c^2=a^2+b^2 \text{.}\)
Solution.
In the above example,
\begin{align*} a=\amp 6\,cm \\ b=\amp 8\,cm \\ c=\amp y \end{align*}
Therefore,
\begin{align*} y^2=\amp (8 cm)^2+ (6 cm)^2 \\ y^2=\amp(8 cm \times 8 cm)+ (6cm \times 6 cm) \\ y^2=\amp 64 cm^2 +36 cm^2 \\ y^2=\amp 100 cm^2 \\ y=\amp \sqrt{100 cm^2} \\ y=\amp 10 cm \end{align*}
Therefore,
side \(y\) is \(10\,cm\)

Example 3.1.3.

Find the length of \(w \) in the figure below.
Solution.
\begin{align*} w^2=\amp (40^2-24^2)\,cm \\ w^2=\amp 1\,600\,cm^2 - 576\,cm^2\\ w^2=\amp 1\,024\,cm^2\\ w=\amp \sqrt{1\,024\,cm^2} \\ w=\amp 32\,cm \end{align*}
Therefore the length of \(w\) is \(32\,cm\)

Example 3.1.4.

In the triangle find the size of line \(PQ \)
Solution.
\begin{align*} PQ^2=\amp (QR^2+PR^2)\,cm \\ PQ^2=\amp (24^2 + 18^2)\,cm^2\\ PQ^2=\amp 576cm^2 + 324\,cm^2\\ PQ^2=\amp \sqrt{900\,cm^2} \\ PQ=\amp 30\,cm \end{align*}
Therefore the length of \(PQ\) is \(30\,cm\)

Example 3.1.5.

The base of a ladder is \(5\,m\) from the base of a vertical wall. A painter places the ladder such that it touches the top of the wall at a point \(12\,m\) above the ground.calculate the length of the ladder (c).
Hint.
  • It is appropriate for the learner to draw the figure as discribed in the question above.
  • Use the pythagorean relationship that is; \(a^2+b^2=c^2\)
Solution.
The figure below shows the sketch of the ladder placced on the wall.
\begin{align*} c^2=\amp(5^2+12^2)\,m^2 \\ c^2= \amp(25+144)\,m^2 \\ c^2=\amp 169 \,m^2 \\ c=\amp \sqrt{169 \,m^2}\\ c=\amp 13 \,m \end{align*}
Therefore,the length of the ladder (c) is \({\color{blue} 13\,m}\text{.}\)