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Geometry Level pending

AB and BC are two equal chords of a circle of length 2 5 2\sqrt { 5 } cm each. If radius of the circle is 5 cm, find the length of the chord AC.

8 cm 6 2 6\sqrt { 2 } 6.7 cm 5 cm 7 cm

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1 solution

Tom Engelsman
Dec 27, 2020

Let O O be the center of the circle with radius equal to 5 5 and with O B A C OB \perp{AC} intersecting at point D . D. Point D D divides O B OB into the lengths: O D = y , D B = 5 y OD = y, DB = 5-y , and it also bisects the chord A C AC into: A D = D C = x . AD = DC = x. Focusing on the two right triangles O D C \triangle{ODC} and B D C \triangle{BDC} , we have the following Pythagorean relationships:

x 2 + y 2 = 5 2 x^2 + y ^2 = 5^2 (for O D C \triangle{ODC} ),

x 2 + ( 5 y ) 2 = ( 2 5 ) 2 x^2 + (5-y)^2 = (2\sqrt{5})^2 (for B D C \triangle{BDC} )

which yield: x = 4 , y = 3. x =4, y = 3. The length of chord A C AC is just equal to A C = 2 x = 2 ( 4 ) = 8 . |AC| = 2x = 2(4) = \boxed{8}.

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