First, we will draw a diagram to understand better the question
so we will use a trigonometric function called the tangent because is the trigonometric function that has as a quotient the opposite leg and the adjacent leg of a triangle in order to obtain how long is the shadow
x is the length of the shadow
[tex]\tan (60)=\frac{15}{x}[/tex]then we need to isolate the x
[tex]x=\frac{15}{\tan (60)}=8.66[/tex]the shadow is 8.66 ft long
