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Which property holds for every equilateral triangle but NOT for every isosceles triangle?

Class 7The Triangle and its PropertiesTriangles
  • AEach of its angles measures 60°.
  • BIt has at least two sides of equal length.
  • CIts three angles add up to 180°.
  • DThe sum of any two of its sides is greater than the third.

Step-by-Step Solution

Only an equilateral triangle is forced to have three 60° angles; an isosceles triangle can have angles such as 100°, 40°, 40°. The other three statements are true of isosceles triangles as well (the last two are true of every triangle), so they cannot separate the two. Trap: choosing "at least two equal sides", which is exactly what the two SHARE, not what distinguishes them.
Answer: A) Each of its angles measures 60°.

Key idea: Angle sum property

**The three interior angles of any triangle sum to 180°.**

This is true for every triangle — equilateral, isosceles, scalene, right, acute, or obtuse. Use it to find a missing angle when you know the other two: third angle = 180° − (first) − (second).

Special cases:
- Equilateral: all angles equal → each is 60°.
- Right triangle: one is 90°, so the other two sum to 90° (they are complementary).

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