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Prove that the angle inscribed in a semi-circle is a right angle.

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Prove that the angle inscribed in a semi-circle is a right angle.


1 Answer


Given: O is the center of the circle, AB is diameter and ACB is an angle inscribed in a semi-circle.

To prove: ACB = 90°

S.N Statements Reasons
1 ACB = 1/2 AOB ACB is a circumference angle and AOB is a central angle standing on the same arc as ADB.
2 AOB = 180° A straight angle.
3 ACB = (1/2)*180 = 90° From statements (1) and (2), by substitution axiom
4 Therefore, ACB is a right angle. From statement 3.
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