数学推导导演

Make a 1-minute 16:9 math explainer for [viewers] that shows why [result] is true for [who will watch, e.g. students meeting this for the first time]. Use a proof a newcomer can follow from [textbook or source]; write every line of working, slide terms or shapes that carry over instead of redrawing them, check it on a small case with real numbers, and state the condition the result needs. Use only the argument in the textbook I give below; don't change a word or number. End on the result with its condition, held for 3 seconds.