bell work: name this polygon. is it regular or irregular?
TRANSCRIPT
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Bell Work:
Name this polygon. Is it regular or irregular?
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Answer:
Hexagon, irregular
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Lesson 20:Triangles
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Triangle: a polygon with three sides.
We classify triangles based on their angle measures and based on the relative lengths of their sides.
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Triangles are classified as acute, right, or obtuse based on the measure of their largest angle.
The measure of the three angles of a triangle total 180 degrees, so at least 2 angles of a triangle are acute.
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Triangles are also classified by relative side lengths.
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Equilateral*: triangles have three equal-length sides. (and three equal angles)
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Isosceles*: triangles have at least two equal-length sides. (and at least two equal angles)
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Scalene*: triangles have sides of different lengths. (and angles of different measures)
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We can use tick marks and arcs to indicate sides and angles that have equal measures.
Any triangle can be classified both by angles and by sides.
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Can an equilateral triangle be an obtuse triangle? Why or why not?
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Answer: No, equilateral triangles have three equal sides and three equal angles that are acute.
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Example:
Classify triangle ABC by angles and sides. A
BC
60°
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Answer:
The triangle is a right triangle and a scalene triangle.
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Example:
The roof of the building in the drawing slopes 35°. What is the measure of the angle at the peak of the roof? 35° 35°
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Answer:
35° + 35° + x = 180°
X = 110°
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Now we will consider how to find the area of a triangle. Triangle ABC is enclosed by a rectangle. What fraction of the area of the rectangle is the area of triangle ABC?
A
BC
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The perpendicular dimensions of a triangle are called the base and height.
Multiplying the base and height of a triangle gives us the area of a rectangle with the same base and height. Since a triangle occupies half the area of the rectangle, we find the area by finding half the area of the rectangle.
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Area of a Triangle
= ½ base x height
= ½ bh
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Example:
Find the area of the triangle.
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Answer:
= ½ (8cm)(5cm)
= ½(40cm )
= 20cm
2
2
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Example:
Find the area of the triangle.
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Answer:
= ½ (4 units)(3 units)
= ½(12 units )
= 6 units
2
2
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HW: Lesson 20 #1-30
Due Next Time