《代数》(必修)(上)(人民教育出版社)有如下例题:例题　把一段半径为Ｒ的圆木,锯成横截面为矩形的木料,怎样锯法才能使横截面面积最大?解　因为锯得的矩形的横截面是圆内接矩形,所以它的对角线是圆的直径,其长度为2Ｒ,设对角线与一条边的夹角为θ(如图1),则ＡＢ=2Ｒｃｏｓθ,Ｂ “Algebra” (required) (above) (People’s Education Press) has the following example: Example: A section of a log with a radius of R is sawn into wood with a rectangular cross-section. How can the sawing method be used to maximize the cross-sectional area? Solution Because The cross-section of the sawed rectangle is a circle inscribed rectangle, so its diagonal is the diameter of the circle, its length is 2R, and the angle between the diagonal and an edge is θ (as in Figure 1), then AB= 2Rcosθ,B