Question

- - 欲做1个低为正方形,容积为108立方米的长方体无盖容器,怎样做法使所用材料最省~

Answer 1

设正方形的边长x,长方体的高为h

则:108=x^2*h

所用材料S=x^2+4x*h=x^2+4x*108/(x^2)=x^2+432/x

=x^2+216/x+216/x>=3*[三次方根下(x^2*216/x*216/x)]

=3*6*6=108

当x^2=216/x时,即:x=6时,所用材料最省