Cyclic Quadrilaterals with Sides 1 and 2

Geometry Level 3

Find the sum of all the unique interior angles (in degrees) in all possible cyclic quadrilaterals that can each be formed with sides of lengths only 1 or 2.


The answer is 630.

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1 solution

David Vreken
Mar 21, 2018

If all the side lengths are the same, then opposite angles must be congruent (as a rhombus) and opposite angles must be supplementary (as a cyclic quadrilateral), and the quadrilateral must then be a square with angles of 90 ° 90° . There are two squares possible, one square with side lengths of 1 1 and one square with side lengths of 2 2 .

If 3 3 of the side lengths are the same and 1 1 is different, then the angles formed by 2 2 congruent side lengths are congruent (since an angle formed by 2 2 chords is congruent to the angle formed by 2 2 chords congruent to the first 2 2 chords), and the angles formed by the 2 2 different side lengths are congruent (since an angle formed by 2 2 chords is congruent to the angle formed by 2 2 chords congruent to the first 2 2 chords) but different from the other congruent angles, and opposite angles must be supplementary (as a cyclic quadrilateral), and the quadrilateral must then be an isosceles trapezoid. Each acute base angle is then cos 1 b a 2 c \cos^{-1}\frac{b - a}{2c} and each obtuse base angle is 90 ° + sin 1 b a 2 c 90° + \sin^{-1}\frac{b - a}{2c} , where a a is the length of the shorter base, b b is the length of the longer base, and c c is the length of the leg of the isosceles triangle. There are two isosceles trapezoids possible, one trapezoid with 3 3 side lengths of 2 2 and 1 1 side length of 1 1 , which would give angles of cos 1 2 1 2 2 75.5 ° \cos^{-1}\frac{2 - 1}{2\cdot2} \approx 75.5° and 90 ° + sin 1 2 1 2 2 104.5 ° 90° + \sin^{-1}\frac{2 - 1}{2\cdot2} \approx 104.5° , and one trapezoid with 3 3 side lengths of 1 1 and 1 1 side length of 2 2 , which would give angles of cos 1 2 1 2 1 = 60 ° \cos^{-1}\frac{2 - 1}{2\cdot1} = 60° and 90 ° + sin 1 2 1 2 1 = 120 ° 90° + \sin^{-1}\frac{2 - 1}{2\cdot1} = 120° .

If there are 2 2 pairs of congruent side lengths and they are alternating, then all the angles formed must be congruent (since an angle formed by 2 2 chords is congruent to the angle formed by 2 2 chords congruent to the first 2 2 chords), and the angles must be supplementary (as a cyclic quadrilateral), and the quadrilateral must then be a rectangle with angles of 90 ° 90° .

Finally, if there are 2 2 pairs of congruent side lengths that are not alternating, then the quadrilateral must be a kite. Also, the 2 2 opposite angles formed by the 2 2 different side lengths must be congruent (since an angle formed by 2 2 chords is congruent to the angle formed by 2 2 chords congruent to the first 2 2 chords), and these two angles must be supplementary (as a cyclic quadrilateral), so these 2 2 angles are 90 ° 90° . The other two angles must then be 2 tan 1 1 2 53.1 ° 2\tan^{-1}\frac{1}{2} \approx 53.1° and 2 tan 1 2 1 126.9 ° 2\tan^{-1}\frac{2}{1} \approx 126.9° .

Therefore, there are 7 unique interior angles: 90 ° 90° , 75.5 ° 75.5° , 104.5 ° 104.5° , 60 ° 60° , 120 ° 120° , 53.1 ° 53.1° , and 126.9 ° 126.9° , and their sum is 630 ° \boxed{630°} .

Because in all cases except squares the unique angles necessarily sum up to 180, we only need to work out 3(180) + 90 = 630. And thus avoid actually having to find the values of the nontrivial unique angles.

Michael Mendrin - 3 years, 1 month ago

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This is true most of the time, but not all of the time. In some cases, the angles in one of the trapezoids can be the same as the angles in the kite, which will make the number of unique angles less than expected. For example, if the side lengths were 1 1 and 2 1 \sqrt{2} - 1 instead, then the angles in the 2 1 \sqrt{2} - 1 , 2 1 \sqrt{2} - 1 , 2 1 \sqrt{2} - 1 , 1 1 trapezoid would be 45 ° 45° and 135 ° 135° , and the angles in the kite would also be 45 ° 45° and 135 ° 135° , making the sum of the unique angles 2 ( 180 ° ) + 90 ° = 450 ° 2(180°) + 90° = 450° .

David Vreken - 3 years, 1 month ago

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Oh, right, that can happen. Well, I did work out the nontrivial angles, and then realized that the sum is 630. But first I did have to find out how many distinct angles there were, which can almost be discerned by careful sketches.

Michael Mendrin - 3 years, 1 month ago

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