The sum of the three angles in a triangle is 180° The sum of the four angles in a quadrilateral is 360°
∠A+∠B+∠C=180°
∠D+∠E+∠F+∠G=360°
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Slide 1: Slide
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This lesson contains 10 slides, with interactive quizzes and text slides.
Lesson duration is: 50 min
Items in this lesson
Sum of the angles
The sum of the three angles in a triangle is 180° The sum of the four angles in a quadrilateral is 360°
∠A+∠B+∠C=180°
∠D+∠E+∠F+∠G=360°
Slide 1 - Slide
Calculating angles
You can use the sum of the angles and your knowledge of special triangles and quadrilaterals to calculate unknown angles.
Slide 2 - Slide
Calculating angles
Calculate the size of angle C.
Slide 3 - Slide
What size is angle C?
A
∠C=360−82−60=218°
B
∠C=82+60=142°
C
∠C=180−82−60=38°
D
∠C=3(82+60)=37°
Slide 4 - Quiz
Calculating angles
The angle sum of a triangle is 180°
∠C=180−82−60=38°
Slide 5 - Slide
Calculating angles
Quadrilateral DEFG is a rhombus. Calculate the size of angle G.
Slide 6 - Slide
What size is angle G?
A
∠G=245°
B
∠G=115°
C
∠G=65°
D
∠G=57.5°
Slide 7 - Quiz
Calculating angles
Quadrilateral DEFG is a rhombus The angle sum of a quadrilateral is 360° A rhombus has rotation and reflection symmetry, therefore the opposite angles are equal.
∠D=∠F=115°
∠G=∠E=2(360−115−115)=2130=65°
Slide 8 - Slide
Equal angles in triangle and quadrilaterals
Isosceles triangle: 2 angles (base angles) are the same size Equilateral triangle: all 3 angles are the same size (60°) Rhombus/Parallelogram: opposite angles are equal Kite: 2 angles are the same size (on either side of the axis of symmetry) Square/Rectangle: all 4 angles are the same size (90°)