Question

An obtuse triangle with area 12 has two sides of lengths 4 and 10. Find the length of the third side. (There are two answers.) (Use law of cosines) ...?

Answer 1

This question can be solved using the Herons equation where 
A = SQRT [s*(s-a)*(s-b)*(s-c)]

A = area of the triangle
a, b and c are the sides of the triangle

s = (a+b+c)/2

Since we are given the area, we can express "s" as a function of the third side c. This can be substituted in the original equation so as to obtain an expression to solve for the third side c

s = (4+10+c)/2 = (14+c)/2

using the solver function of the calculator or MS Excel, the third side is 7.21 units