https://zh.berikutyang.com/post/the-attached-picture-shows-the-situation-where-two-boards-a-and-b-are-placed-on-the-ground-in-the-figure-1-and-2-are-the-angle-between-the-two-boards-a-and-b-and-the-ground-and-3-is-the-angle-between-the-two-boards-if