A triangle is a plane figure bounded by three straight lines. A triangle has three sides and three angles, and each one of them is called an element of the triangle.


Here, PQR is a triangle, its three sides are line segments PQ, QR and RP; ∠PQR, ∠QRP and ∠RPQ are its three angles.

The point at which any two sides meet is called a vertex. Thus, a triangle has three vertices. The vertices of the triangle PQR are P, Q and R.

The side opposite to a vertex is called a base.

The above figure of a triangle PQR, QR is the base when the vertex is P. 

Exterior opposite angles:

If a side QR of a triangle PQR is produced to S then the ∠PRS is called the exterior angle at R.

Exterior and Interior Opposite Angles

Interior opposite angles:

The two opposite angles, that is ∠PQR and ∠QPR are called interior opposite angles.


The sum of the three sides of a triangle is called the perimeter of the triangle. 


The perimeter of the triangle PQR is (PQ + QR + RP)

9th Grade Math

From Triangle to HOME PAGE

Didn't find what you were looking for? Or want to know more information about Math Only Math. Use this Google Search to find what you need.

New! Comments

Have your say about what you just read! Leave me a comment in the box below. Ask a Question or Answer a Question.

Share this page: What’s this?

Recent Articles

  1. Shifting of Digits in a Number |Exchanging the Digits to Another Place

    May 19, 24 06:35 PM

    Shifting of Digits in a Number
    What is the Effect of shifting of digits in a number? Let us observe two numbers 1528 and 5182. We see that the digits are the same, but places are different in these two numbers. Thus, if the digits…

    Read More

  2. Formation of Greatest and Smallest Numbers | Arranging the Numbers

    May 19, 24 03:36 PM

    Formation of Greatest and Smallest Numbers
    the greatest number is formed by arranging the given digits in descending order and the smallest number by arranging them in ascending order. The position of the digit at the extreme left of a number…

    Read More

  3. Formation of Numbers with the Given Digits |Making Numbers with Digits

    May 19, 24 03:19 PM

    In formation of numbers with the given digits we may say that a number is an arranged group of digits. Numbers may be formed with or without the repetition of digits.

    Read More

  4. Arranging Numbers | Ascending Order | Descending Order |Compare Digits

    May 19, 24 02:23 PM

    Arranging Numbers
    We know, while arranging numbers from the smallest number to the largest number, then the numbers are arranged in ascending order. Vice-versa while arranging numbers from the largest number to the sma…

    Read More

  5. Comparison of Numbers | Compare Numbers Rules | Examples of Comparison

    May 19, 24 01:26 PM

    Rules for Comparison of Numbers
    Rule I: We know that a number with more digits is always greater than the number with less number of digits. Rule II: When the two numbers have the same number of digits, we start comparing the digits…

    Read More