Lesson 5.25.2 Small Group · Group 1
Start hereWords, worked example, and sentence starters
Learning target With my small group, I can find the area of a triangle using the formula A = ½ × base × height — one step at a time, with support.
Start hereWords for this lesson
Read each word before you begin. Say it out loud.
| Word | What it means | Example |
|---|---|---|
| BaseSpanish: 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. | b in A = ½bh |
| HeightSpanish: Altura | The straight-up distance from the base to the top corner. | h in A = ½bh |
| AreaSpanish: Área | How much space is inside a flat shape. | measured in cm², in², ft² |
| PerpendicularSpanish: Perpendicular | Two lines that meet to make a square corner (90 degrees). | The corner of a book or a door frame — the edges meet at exactly 90°, shown by a small square symbol |
| Composite figureSpanish: Figura compuesta | A shape made by putting two or more simple shapes together. | A house shape = a rectangle (the walls) + a triangle (the roof); total area = rectangle area + triangle area |
| FormulaSpanish: Fórmula | A math rule written with symbols. | A = ½ × b × h means Area equals one-half times base times height; for b = 12 and h = 8, A = 48 |
How it worksWorked examplefinding the area of a triangle
These numbers are not on your problems. The steps are. Follow them with your own numbers.
A triangle has a base of 16 cm and a perpendicular height of 9 cm. Find the area.
- Two copies of a triangle form a parallelogram, so a triangle is half of one.
- Write the formula. A = ½ × b × h
- Name the parts. b = 16 cm h = 9 cm, meeting the base at a square corner
- Multiply, then halve. 16 × 9 = 144, and half of 144 is 72
Answer: The area of the triangle is 72 square centimeters (72 cm²).
Now you trySame steps, your turn
The steps are the same as the worked example. The numbers are yours. Do the work.
A triangle has a base of 14 in. and a perpendicular height of 7 in. Find the area.
- A triangle is half of what shape?
- Write the formula. A = ½ × ×
- Base = in. Perpendicular height = in.
- Multiply, then halve. × = , half is
Say it and write itSentence starters
Finish each sentence out loud with a partner. Then use them in your writing.
- The base of the triangle is and the height is .
- I know that measure is the height because .
- Two copies of this triangle would compose a .
- Area = ½ × × = square units.
Word bank triangleparallelogramhalfbaseheightperpendicularsquare cornerareaformularight trianglecopyrotate
Watch outA common mistake
A common mistake in Area of Triangles is forgetting the ½ — students multiply base × height like a rectangle and stop there, or they use the triangle's slanted side instead of the perpendicular height that's already given. Always check: did I take half, and did I use the straight-up height, not the slanted side?
Lesson 5.25.2 Small Group · Group 1
Group 1Supported practice
Learning target With my small group, I can find the area of a triangle using the formula A = ½ × base × height — one step at a time, with support.
-
1Complete each step, then write the final answer.
The height goes from the top corner straight down. A triangle has a base of 14 cm and a height of 6 cm. Find its area.
- 1Multiply the base by the height. What do you get?
- 2Now take half. What is half of 84?
- 3What units does the area use?
Final answer: -
2Complete each step, then write the final answer.
The slanted side is longer than the height. A triangle has a base of 16 in and a height of 9 in. One slanted side is 11 in. Find its area.
- 1Which length is the height: 9 in or 11 in?
- 2Multiply the base by the height.
- 3Take half. What is the area?
Final answer: -
3Complete each step, then write the final answer.
A triangle has a base of 6 ft and a height of 5 ft. Find its area.
Show your workHint Multiply the base by the height. Then take half.
Final answer: -
4Complete each step, then write the final answer.
A triangle has a base of 18 m and a height of 11 m. Find its area.
Show your workHint Find 18 × 11 first. Then take half of it.
Final answer: -
5Complete each step, then write the final answer.
A sail is shaped like a triangle. Its base is 9 feet. Its height is 14 feet. What is the area of the sail?
Show your workHint Use A = 1/2 × b × h. Multiply first, then take half.
Final answer: -
6Complete each step, then write the final answer.
Sam says a triangle with a base of 10 in and a height of 7 in has an area of 70 square inches. Is he right? What is the area?
Show your workHint Sam found the rectangle. What part of the rectangle is the triangle?
Final answer:
Explain your thinkingWhy do we take half of base × height for a triangle? Use Sam’s mistake to explain.
Sentence starter Base × height gives the area of a ___. · The triangle is half because ___.