Lesson 2: Determine the Area of Triangles6.GR.1

I can…

I can find the area of a triangle using the formula A = ½ × base × height.

Key words

• Area of a Triangle — The space inside a triangle. A triangle is half of a rectangle with the same base and height, so A = ½ × b × h.
• Base — Any side of a triangle can be the base. Once you pick a base, the height must be measured perpendicular (at a 90° angle) to it.
• Height — The straight-up distance from the base to the top corner.
• Area — How much space is inside a flat shape.
• Perpendicular — Two lines that meet to make a square corner (90 degrees).
• Composite figure — A shape made by putting two or more simple shapes together.
• Formula — A math rule written with symbols.
• Parallel — Two lines or sides that stay the same distance apart and never meet.
• Substitute — Put the numbers you know into a formula.

Steps

1. My triangular garden has a base of 12 feet and a height of 8 feet.
2. I write the formula: A = ½ × base × height.
3. I put in the numbers: A = ½ × 12 × 8.
4. I multiply base × height first: 12 × 8 = 96.
5. I take half: ½ × 96 = 48. So the area is 48 square feet.

Try it

A triangular flag for a family picnic has a base of 9 inches and a height of 6 inches. What is its area?

Check your answer

27 sq in

A = 1/2 × 9 × 6 = 1/2 × 54 = 27 square inches.

Sentence starter

I know ___ because ___.

Goal: I can explain my steps using the words base, height, area, and perpendicular.