Justifying the steps
Types of exercises: justifying the steps - angle facts.
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Which reason fits this step?
Three angles share a vertex, do not overlap and together fill a full turn. Two of them are 90° and 145°. How many degrees is the third? A pupil wrote: 360° − 90° − 145° = 125°
Which reason fits this step?
In triangle ABC, angle A is 45° and angle B is 30°. How many degrees is angle C? A pupil wrote: C = 180° − 45° − 30° = 105°
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Two lines cross. Angles α and β are vertically opposite, and angle α is 95°. How many degrees is angle β? A pupil wrote: β = 95°
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Lines k and l are parallel, and line m crosses them. An angle at line k is 40°. How many degrees is the corresponding angle at line l? A pupil wrote: 40°
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Triangle ABC is isosceles with base AB. Angle A is 55°. How many degrees is angle B? A pupil wrote: B = 55°
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Angles α and β lie next to each other on a straight line. Angle α is 50°. How many degrees is angle β? A pupil wrote: β = 180° − 50° = 130°
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In quadrilateral ABCD, angles A, B and C are 80°, 95° and 120°. How many degrees is angle D? A pupil wrote: D = 360° − 80° − 95° − 120° = 65°
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Triangle ABC is isosceles with base AB. Angle A is 80°. How many degrees is angle B? A pupil wrote: B = 80°
Which reason fits this step?
Three angles share a vertex, do not overlap and together fill a full turn. Two of them are 85° and 105°. How many degrees is the third? A pupil wrote: 360° − 85° − 105° = 170°
Which reason fits this step?
In triangle ABC, angle A is 40° and angle B is 45°. How many degrees is angle C? A pupil wrote: C = 180° − 40° − 45° = 95°
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