Question 201722
watch the animation here using a piece of string between 2 points. http://www.mathopenref.com/constellipse1.html

the effective base length (b) of the triangle would be the straight line distance between the 2 end points of the sting and the sum of the 2 remaining sides (c) is equal to the total length of the sting, as the stirng does not change length.

Hope that helped 
Adam