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Key Points About Quadrilaterals

1. Sum of the angles of a quadrilateral is 360°.

2. A diagonal of a parallelogram divides it into two congruent triangles.

3. In a parallelogram,

(i) opposite sides are equal

(ii) opposite angles are equal

(iii) diagonals bisect each other

4. A quadrilateral is a parallelogram, if

(i) opposite sides are equal or

(ii) opposite angles are equal or

(iii) diagonals bisect each other or

(iv) a pair of opposite sides is equal and parallel

5. Diagonals of a rectangle bisect each other and are equal and vice-versa.

6. Diagonals of a rhombus bisect each other at right angles and vice-versa.

7. Diagonals of a square bisect each other at right angles and are equal, and vice-versa.

8. The line-segment joining the mid-points of any two sides of a triangle is parallel to the third side and is half of it.

9. A line through the mid-point of a side of a triangle parallel to another side bisects the third side.

10. The quadrilateral formed by joining the mid-points of the sides of a quadrilateral, in order, is a parallelogram.

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FAQs on Key Points About Quadrilaterals, Class 9, Maths

1. What are the key points to remember about quadrilaterals in class 9 mathematics?
Ans. The key points to remember about quadrilaterals in class 9 mathematics are: - A quadrilateral is a polygon with four sides and four vertices. - The sum of the interior angles of a quadrilateral is always 360 degrees. - There are different types of quadrilaterals such as parallelograms, rectangles, squares, rhombuses, and trapezoids. - In a parallelogram, opposite sides are parallel and opposite angles are equal. - In a rectangle, all angles are right angles. - In a square, all sides are equal and all angles are right angles. - In a rhombus, all sides are equal but the angles are not necessarily right angles. - In a trapezoid, only one pair of opposite sides are parallel.
2. What is the sum of the interior angles of a quadrilateral?
Ans. The sum of the interior angles of a quadrilateral is always 360 degrees. This means that if we measure the angles of a quadrilateral and add them together, the total will always be 360 degrees.
3. What are the different types of quadrilaterals?
Ans. There are different types of quadrilaterals, including: - Parallelogram: In a parallelogram, opposite sides are parallel and opposite angles are equal. - Rectangle: In a rectangle, all angles are right angles. - Square: In a square, all sides are equal and all angles are right angles. - Rhombus: In a rhombus, all sides are equal but the angles are not necessarily right angles. - Trapezoid: In a trapezoid, only one pair of opposite sides are parallel.
4. How do we determine if a quadrilateral is a parallelogram?
Ans. To determine if a quadrilateral is a parallelogram, we need to check if its opposite sides are parallel and if its opposite angles are equal. If both of these conditions are satisfied, then the quadrilateral is a parallelogram.
5. What are the properties of a square?
Ans. The properties of a square include: - All sides of a square are equal in length. - All angles of a square are right angles. - The diagonals of a square are equal in length and bisect each other at right angles. - The diagonals of a square divide it into four congruent right-angled triangles. - The perimeter of a square is four times the length of its side. - The area of a square is the square of its side length.
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