Hi there!
Since ABC is an equilateral triangle, all angles of the triangle would be 60 degrees. Triangle ADC and DAC are both right triangles because they have right angles. The length of BD is 5 because BD is half of the side, which has a total of 10. Using pythagorean theorem to find the length of AD, here's what we'd do: 10^2 - 5^2 = AD^2 | 100 - 25 = AD^2 | 75 = AD^2 | AD = 8.66. Triangle ADB is a right triangle because it has a right angle. AB is double BD, or BD is half of AB. BD is 3.66 units less than AD.
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