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众所周知,利用初等方法求最值,除了纯几何方法外,通常利用函数.(一次函数或二次函数)的性质,基本不等式或三角代换外,还可以就是利用一元二次方程根的判别式Δ=b~2-4ac.而利用Δ求最值往往会涉及如何选主元的问题.请看以下两例:例1 如图1,已知甲在河沿AC的A处,忽见对岸B处有一小孩落水呼救.甲立刻赶去抢救,已知河
As we all know, the use of elementary methods to find the most value, in addition to pure geometric methods, usually use the property of the function (primary or quadratic), basic inequality or trigonometric substitution, and can also be a discriminant using the root of a quadratic equation. Δ=b~2-4ac. The use of Δ to find the best value often involves the question of how to select the pivot element. Look at the following two examples: Example 1 As shown in Figure 1, A is known at the A of the riverside AC and suddenly sees the opposite bank B. A child fell into the water to call for help. A immediately rushed to rescue the river.