Step 1: Observe the given figure and make a note of an important points

Step 2: Find the right angle triangles in the given picture and apply the Pythagoras theorem.

              EXAMPLE: ΔABC is a right triangle, so by the Pythagorean theorem,

                               AB^2+BC^2 = AC^2 ..........................(1)

                                ΔABE is a right triangle

                                  so, AB^2+BE^2 = AE^2...........................(2)

                                  ΔDBC is a right triangle

                                So, DB^2+BC^2 = CD^2............................(3)

                                    ΔDBE is a right triangle

                            So, DB^2 + BE^2 = DE^2...............................(4)

Step 3: Add BE^2 + DB^2 to both sides of equation (1)

            EXAMPLE: AB^2 + BC^2 + BE^2 + DB^2 = AC^2 + BE^2 + DB^2

                             Rearrange the terms:

                   AB^2 + BE^2 + DB^2 + BC^2 = AC^2 + DB^2 + BE^2

                        Let's put parentheses

                  (AB^2 + BE^2) + (DB^2 + BC^2) = AC^2+(DB^2+BE^2)

                     AE^2 + CD^2 = AC^2 + DE^2

                                                [ \because   Equation (2), (3) and (4)]