The surface area is the area of the rectangular base (length width) plus the areas of each of the four triangular faces. The area of a triangle is 1/2 base height so all that remains is to find the height of each of the triangular faces. I am going to show you how to find he height of the triangular face PQR in the diagram.In the diagram C is the centre of the rectangular base of the pyramid, S is the centre of the side QR of the base, h is the height of the pyramid and x is the length of the side of the base as shown. Thus the triangle PCS is a right triangle with side lengths h and x/2. Thus you can use Pythagoras theorem to find the length of the hypotenuse PS which is the height of the triangle PQS. Use this height to find the area of the face PQR.
