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20 17 Hefei three-mode answer mathematics
② The plane PQR is cut into cubes and the cross-sectional figure is an isosceles trapezoid;

Mistake.

Do RM parallel PQ, across BB 1 to m; Make PN parallel to QR and pass through A 1D 1 in n.

The pentagon formed by five points PQRMN is the section.

The best way to find a section in a cube is to do parallel operation and find the intersection point of each edge.

④ the line segments EF and GH move on the sides A 1B 1 and CC 1 respectively, and EF+GH= 1, then the triangular pyramid e

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The maximum volume of FGH is112.

Let g be above h, then E-FGH=G-EFC 1 minus H-EFC 1.

= 1/3 * EF * GC 1- 1/3 * EF * HC 1

= 1/3*EF*GH

EF+GH= 1, then the maximum value of EF*GH = 1/4.

This item is correct.