9.1.1 Developing Formulas for Triangles and Quadrilaterals (continued)
The area of a trapezoid is determined in this example. The lengths of the bases and the height are given. To calculate the area, substitute the given values into the formula for the area of a trapezoid. Simplify and solve for area. Remember that the units for area are square units, in this case square centimeters (cm 2 ).
The area of a triangle with base b and height h is A bh . = This formula would be the formula for half the area of a parallelogram formed by two congruent triangles. To visualize why this is the case, add an inverted congruent triangle to one end of a triangle and see that the area of the parallelogram formed would be A = bh , or twice the area for one of the triangles. The length of the base of a triangle is determined in this example. The height and the area of the triangle are given as algebraic expressions. 1 2 To calculate the base length, substitute the given values into the formula for the area of a triangle. Isolate the term for the base, simplify, and solve for the base length. The length is also an expression with the unknown x . The units are feet.
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