where did the proportions to the sides of the triangle formed to find joint B forces come from (5,12,13)

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Diagram: IS k B Cy Cx 20 A 1) Determine the horizontal force acting on the frame at A (the answer is in kips and the sign is according to the Cartesian coordinate system). 2) Determine the vertical force acting on the frame at A (the answer is in kips and the sign is according to the Cartesian coordinate system). 3) Determine the horizontal force acting on the frame at D (the answer is in kips and the sign is according to the Cartesian coordinate system). 4) Determine the vertical force acting on the frame at D (the answer is in kips and the sign is according to the Cartesian coordinate system). 5) Determine Cx (the answer is in kips and the sign is according to coordinate system in the diagram). 6) Determine Cy (the answer is in kips and the sign is according to coordinate system in the diagram).

Determine degree of static indeterminacy of truss