Geometry and TrigonometryEasyTriangle angle sum
Geometry and Trigonometry practice question
Two angles of a triangle measure 40° and 75°. What is the measure of the third angle?
- A. 55°
- B. 65°Correct
- C. 75°
- D. 115°
Answer: B. 65°
The angles of a triangle sum to 180°. So the third angle = 180 − 40 − 75 = 65°. Choice D (115) is 40 + 75 — the sum of the other two, not the remaining angle.
Why the other answers are wrong
- A. 55°
- 55° isn't produced by any standard slip here — it's just a plausible-looking decoy. Set up 180 − 40 − 75 and subtract carefully to get 65°.
- C. 75°
- 75° just repeats one of the angles you were given; the third angle is what's left over: 180 − 40 − 75 = 65°.
- D. 115°
- 115° is 40 + 75, the sum of the two known angles. You still need to subtract that from 180° to find the remaining angle.
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