What is the approximate length of the base of an isosceles triangle if the congruent sides are 3 feet and the vertex angle is 35°?

 1.80 feet.

Step-by-step explanation:          

Please see the attachment.

Let c be the length of base of our given triangle.

We have been given that an isosceles triangle's congruent sides are 3 feet the vertex angle is 35 degrees. We are asked to find the length of the base of isosceles triangle.      

We will use law of cosine to find the length of our base.

Upon substituting our given values in above formula we will get,

Therefore, the length of base of our given isosceles triangle is approximately 1.80 feet.