在数学教学中,教师经常要编选部分练习题,这对教学工作是有益的。下面举例说明我是怎样利用平行投影来拟造数学题的。将任意△ABC平行投影到x轴上,设BC与x轴的夹角为∠xDE=θ(图1),则CA与x轴的夹角为∠xFA=θ-C-180°,AB与x轴的夹角为∠xGH=θ+B-180°,而把封闭折线BCAB平行正投射到x轴上,于是有BCcosθ+CAcos(θ-C-180°)+ABcos(θ+ In mathematics teaching, teachers often need to edit some exercises, which is beneficial to teaching. Here’s an example of how I used parallel projections to make math problems. Arbitrary △ABC is projected on the x-axis in parallel. Let the angle between BC and the x-axis be ∠xDE=θ (Fig. 1), then the angle between CA and the x-axis is ∠xFA=θ-C-180°, AB and The included angle of the x-axis is ∠xGH=θ+B-180°, and the closed polyline BBCB is projected parallel to the x-axis, so there is BCcosθ+CAcos(θ-C-180°)+ABcos(θ+