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We know that area of the trapezium ABCD whose two parallel sides are AB and DC and the distance between parallel sides is h, is given by
Let ABC be a triangle with vertices and
as shown in figure below.
Draw the ordinates AL, CM and BN.
Note that area of triangle ABC = Area of trapezium ALNB + Area of trapezium ACML - Area of trapezium BCMN
=
But
|
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How to obtain Area of a Triangle?
Rules to write down the area of a triangle whose vertices are given: (i) Write down the coordinates of the vertices in order in two columns, repeating at the end the coordinates at the head, (ii) draw diagonal arrows as in cross multiplication, and draw two vertical lines to enclose the two columns, and (iii) prefix the factor Simplification Take the products of each row and the next crossway, putting the minus sign between them, as in cross multiplication, add the results, and multiply the sum by |
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Sign of Area: The areas of the triangles ABC and ACB are opposite in sign. In numerical cases, we usually ignore the sign of the area and take the absolute value of the area of the triangle as the final result.
Thus, we should write as follows:
That is, brackets in formula (1) have been replaced by the absolute value sign, ,
Illustration: Find the area of the triangle whose vertices are (3,8),(7,2) and (-1,1).
Solution: The area of the triangle whose vertices are (3,8),(7,2),(-1,1) is
= 26 sq. units