The measure of the diagonal KI is approximately 6.465 centimeters.
The diagonals of the quadrilateral RISK are represented by line segments RS and KI, both measured in centimeters. There are two triangles forming the quadrilateral: [tex]\triangle RIS[/tex], [tex]\triangle RKS[/tex]
By Law of Sine and Law of Cosine we determine the measures of all internal angles:
Triangle RIS
[tex]RS^{2} = RI^{2}+IS^{2}-2\cdot RI\cdot IS \cdot \cos I[/tex] (1)
[tex]\frac{RS}{\sin I} = \frac{RI}{\sin S'} = \frac{IS}{\sin R'}[/tex] (2)
Triangle RKS
[tex]RS^{2} = RK^{2} + KS^{2}-2\cdot RK\cdot KS\cdot \cos K[/tex] (3)
[tex]\frac{RS}{\sin K} = \frac{RK}{\sin S''} = \frac{KS}{\sin R''}[/tex] (4)
[tex]S = S' + S''[/tex] (5)
[tex]R = R' + R''[/tex] (6)
By (1) and (3) we find the values of angles I and K:
[tex]I = \cos^{-1}\left(\frac{RI^{2}+IS^{2}-RS^{2}}{2\cdot RI\cdot IS} \right)[/tex]
[tex]I \approx 90.261^{\circ}[/tex]
[tex]K = \cos^{-1}\left(\frac{RK^{2}+KS^{2}-RS^{2}}{2\cdot RK\cdot KS} \right)[/tex]
[tex]K\approx 94.903^{\circ}[/tex]
By (2) and (4) we find the remaining angles for each triangle:
[tex]S' = \sin^{-1}\left[\left(\frac{RI}{RS} \right)\cdot \sin I\right][/tex]
[tex]S' \approx 38.682^{\circ}[/tex]
[tex]R' = \sin^{-1}\left[\left(\frac{IS}{RS} \right)\cdot \sin I\right][/tex]
[tex]R' \approx 51.057^{\circ}[/tex]
[tex]S'' = \sin^{-1}\left[\left(\frac{RK}{RS} \right)\cdot \sin K\right][/tex]
[tex]S'' \approx 28.969^{\circ}[/tex]
[tex]R'' = \sin^{-1}\left[\left(\frac{KS}{RS} \right)\cdot \sin K\right][/tex]
[tex]R'' \approx 56.128^{\circ}[/tex]
Then, we have the values of angles R and S by (5) and (6):
[tex]S \approx 67.651^{\circ}[/tex], [tex]R \approx 107.185^{\circ}[/tex]
The measure of the diagonal KI is found by the Law of Cosine:
[tex]KI = \sqrt{IS^{2}+SK^{2}-2\cdot IS\cdot SK\cdot \cos S}[/tex]
[tex]KI \approx 6.465\,cm[/tex]
The measure of the diagonal KI is approximately 6.465 centimeters.
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