Trigonometry For Dummies
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You can find the area of a triangle using Heron's Formula. Heron's Formula is especially helpful when you have access to the measures of the three sides of a triangle but can't draw a perpendicular height or don't have a protractor for measuring an angle.

Consider the situation where you have a large ball of string that's 100 yards long and you're told to mark off a triangular area — with the string as the marker for the border of the area. You walk 40 yards in one direction, take a turn, and walk another 25 yards; then you head back to where you started and use up that last 35 yards of string.

How large an area have you created?

Heron's Formula reads:

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where a, b, and c are the lengths of the sides of the triangle and s is the semi-perimeter (half the perimeter).

In the case of your triangle and the string, the perimeter is 40 + 25 + 35 = 100 yards. Half that is 50, so the formula now reads:

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You've marked off an area of approximately 433 square yards.

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Mary Jane Sterling is the author of Algebra I For Dummies and many other For Dummies titles. She has been teaching mathematics at Bradley University in Peoria, Illinois, for more than 30 years and has loved working with future business executives, physical therapists, teachers, and many others.

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