学数学,既要善于抓住不变的根本,又要善于灵活地在变化中认识、处理和解决问题。三角形的内角和定理及其推论常常是几何问题中的隐含条件,合理和灵活地应用它们,也常常能使几何题达到一题多解和一题多变的效果。图1一、一题多解例如图1,E为△ABC内一点,求证:(1)∠AEB=∠1+∠2+? To learn mathematics, we must be good at grasping the fundamentals of change, but also be good at flexibly understanding, dealing with, and solving problems in change. The internal angles of triangles and theorems and their inferences are often implicit conditions in geometric problems. Applying them rationally and flexibly can often make geometric problems achieve multiple solutions to a problem and a variable effect. Figure 1, a multi-solution to a problem such as Figure 1, E is △ ABC within a point, verify: (1) ∠ AEB = ∠ 1 + ∠ 2+ ??