The difficulty of mathematics in the first volume of the eighth grade of Tianxingjin examination paper is caused by the difficulty and depth of topic design.

1. Difficult topic design: The topic design team of Tianxingjin test paper focuses on examining students' understanding and application ability of mathematical knowledge, and the topic involves more complex mathematical concepts, reasoning and problem-solving ability. These topics require students to have higher mathematical thinking and logical thinking ability.

2. Depth of the topic: The topic of Xingjin test paper has a certain depth, which requires students to think, reason and summarize in the process of solving problems. This kind of topic design can cultivate students' comprehensive thinking ability and problem-solving ability.