Subtract to Find an Unknown Angle Part
Write whole = known + unknown.
About 20 minutes, plus any extended practice
Supplied materials
- Paper and a pencil; drawing and spoken explanations are welcome. An adult may read directions and options aloud.
- Supplied worked-case record: A straight angle has adjacent parts 112° and an unknown.
- All quantities and shape descriptions are supplied in the worked example and questions; no purchase or personal information is needed.
Learn the idea
A known whole angle can be split into a measured part and a missing part. Subtract the known part from the whole. The result must be nonnegative and should recombine with the known measure to reproduce the whole. Use supplied angle relationships rather than judging a sketch by eye.
Write whole = known + unknown. Subtract the known measure and check by adding both parts. A right angle supplies ninety degrees, and a straight angle supplies one hundred eighty when those conditions are explicitly stated.
Name the stated whole-angle benchmark before subtracting. Do not assume a sketch is exactly to scale. Label the entire angle and its two adjacent parts with a missing-value symbol.
- Label the entire angle and its two adjacent parts with a missing-value symbol.
- Follow the worked example. Point to each number or shape feature and say what it tells you.
- Write whole = known + unknown.
- Try the question and then the three practice questions. Check your answers and explain any change you make.
See the method in action
A straight angle has adjacent parts 112° and an unknown. Since its whole is 180°, the missing part is 68°. Checking 112+68=180 verifies the measure. A roughly drawn smaller-looking part does not give an exact answer without the stated whole relationship. Name the stated whole-angle benchmark before subtracting. Do not assume a sketch is exactly to scale. Label the entire angle and its two adjacent parts with a missing-value symbol.
Check your reasoning
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Helpful tips
- Name the stated whole-angle benchmark before subtracting.
- Do not assume a sketch is exactly to scale.
See the answer and explanation
65
Subtract the known part from the right angle’s 90 degrees. The result is 65.
Take it further
A straight angle with one 73° part has a 107° remaining part.
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More practice: try new cases
Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.
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