人教课标版试验教材八年级《数学》下册101页第12题是一道错误的习题,为了让读者对此题有一个清晰的认识,不妨将这道习题及其配套的《教师教学用书》的解法转载如下:“阳光透过矩形玻璃窗投射到地面上,地面上出现了一个明亮的四边形,小刚用量角器量出这个四边形的一个锐角恰好? The teaching materials of the textbooks for the eighth grade mathematics, the second volume of 101 pages, the 12th item is an erroneous exercise. In order for the reader to have a clear understanding of this topic, it is worthwhile to include this exercise and its supporting “teaching books for teachers”. The solution to the question is reproduced as follows: "The sunlight is projected onto the ground through a rectangular glass window. A bright quadrangle appears on the ground. Xiaogang protractor measures the acute angle of this quadrilateral.