Question

Area of isosceles tringle if base is 6 cm and perimeter is 26 cm

Answer 1

I don't know how the army does this. Trig? formula.
The formula using straight algebra is
A = sqrt(p(p-a)(p-b)(p-c) where
p is 1/2 the perimeter and a,b,c are the sides.
The perimeter is 26 and the base is 6 which makes the other two sides 26-6 = 20 and since it is an isosceles triangle the other two sides must be 10 each.
A in cm^2 = sqrt[13(13-10)(13-10)(13-6)]

Answer 2

let each of the equal sides be x
2x + 6 = 26
x = 10

make a sketch of the triangle, draw in its height h
h^2 + 3^2 = 10^2
h = √91

area = (1/2)(6)√91 = 3√91 cm^2

which is the same as DrBob's answer