Answer:
Yes, it is possible
Step-by-step explanation:
In a triangle, the length of the longest side of the triangle cannot be larger than the sum of the lenghts of the two shortest sides.
In formula, this means that:
where
c is the length of the longest side
a, b are the lengths of the shortest side
In this problem, we want to build a triangle using 3 pieces of lumber, each of length:
a = 3 ft
b = 7 ft
c = 8 ft
We can verify that the length of the longest piece of lumber is less than the sum of the lengths of the other two sides, so:
Therefore, yes, it is possible to build a closed triangular pen.