Achild designs a flag in the shape of a rhombus, as shown in the diagram below. which expression can be used to determine the side length of the rhombus?

Which expression can be used to determine the side length of the rhombus?

 The answer is the first option: 10/Cos(30°)

 Explanation:

 1. As you can see in the figure attached, there is a right triangle and its hypotenuse (which is represented with the letter "x") is the side you want to calculate. So, you have:

 Cos(α)=Adjacent leg/Hypotenuse

 α=30°
 Adjacent leg=(20 in)/2=10 in
 Hypotenuse=x

 2. When you substitute these values into the formula, you obtain:

 Cos(α)=Adjacent leg/Hypotenuse
 Cos(30°)=10/x

 3. Therefore, you obtain the expression to determine the side length of the rhombus by clearing the hypotenuse "x", as below:

 x=10/Cos(30°)