Determine the ratios of the weights of four beams that have same length.are made of the same material,are subjected to the same lmaximum bending moment,and have the same maximum bending stress if...

Determine the ratios of the weights of four beams that have same length.are made of the same material,are subjected to the same lmaximum bending moment,and have the same maximum bending stress if their cross sections are (I) a rectangle with height equal to twice the width,(2) a square,(3) a circle , and (4) a pipe with outer diameterd
and wall thickmessf
=
di%(see figures).