Theorem: All the three interior angles of an equilateral triangle are equal to 60^o
Prerequisites:
Equilateral triangle (definition)
Isosceles triangle theorem (proof)
Angle sum property of a triangle (proof)
Proof:
Let \triangle ABC be an equilateral triangle. Then by the definition of an equilateral triangle, all the sides are equal.
Since AB = AC, hence by using the isosceles triangle theorem,
\qquad\quad\angle B = \angle C
Similarly, since AB = BC, therefore
\qquad\quad\angle A = \angle C
\therefore\quad\;\;\;\angle A = \angle B = \angle C = \theta\qquad\qquad\qquad\qquad\cdots\text{(1)}
Further, by the angle sum property of a triangle,
\qquad\quad\angle A +\angle B + \angle C= 180^o
\Rightarrow\quad\;\; 3\theta = 180^o\qquad\qquad\qquad\qquad\qquad\qquad\;\;\text{(from $(1)$)}\\
\Rightarrow\quad\;\; \theta = 60^o
Q.E.D.
Recommended:
Converse of Isosceles Theorem
Pythagoras Theorem
Basic Proportionality Theorem
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