Step 1: Make a note of the known and unknown measurements

Step 2: Use formula of area of isosceles triangle to write 

            FORMULAS:

              Area of an isosceles triangle = \frac{1}{2} b*h

              Where b - base and h - height (altitude)

                   h (Altitude) = \sqrt{a^2 - \frac{b^2}{4}}

Step 3: Substitute the known values in the area formula.

            EXAMPLE:  Area of an isosceles triangle = \frac{1}{2} b*h

                                              20 = \frac{1}{2} 10 * h

Step 4: Simplify the equation and find the unknown value (Height)

              EXAMPLE : h = \frac{20}{5}

                                    h = 4

Step 5: Set up a formula to calculate the lateral side of an isosceles triangle.

              NOTE: Isosceles triangle is a combination of two right triangle

                            Pythagoras theorem

              FORMULA:(lateral\ side)^2=(height)^2+\left(\frac{base}{2}\right)^2  

Step 6:  Find the lateral side of an isosceles triangle.

            EXAMPLE: (lateral\ side)^2=4^2+(\frac{10}{2})^2

                                lateral\ side=\sqrt{16+25}          

                                lateral\ side=\sqrt{41}