17. A cylindrical kite frame is to be constructed from a 4-m length of light bendable rod. The
frame will be made up of two circles joined by
four straight rods of equal length. In order to
maximize lift, the kite frame must be con-
structed to maximize the volume of the cylinder.
Into what lengths should the pieces be cut in
order to optimize the kite's flight?

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.

goog may have it for you