缺“1”就错
在不等式求最小值中,常数“1”的魅力非常的大,通过“1”的中介,可以帮助避免误区,获得成功。
一 “1”在整体中的应用
例:已知,且,求的最小值。
解 常见误区:,
的最小值是12
误区分析:时,当时,取得等号;又因为取到等号时既;出现与的矛盾
正确突击:
当时,既 时取到等号,
时,的最小值是16
二 把分子换成“1”
例:已知且,求的最小值。
解 常见误区:
误区分析: 时,当 既取到等号,又因为时,当,既时取到等号,与矛盾。
正确突击:
当,既时,的最小值是9
三 在待求式中应用“1”
例:若,求的最小值。
解 常见误区:
误区分析:要使取到等号,所以既时;但由时取到等号,所以既时取到等号;所以与 不能同时取到等号。
正确突击:
当,既,时的最小值 是
实战沙场:
1 已知且,求的最小值。(参考答案:时; 的最小值是9)
2 已知且,求的最小值。(参考答案:时;的最小值是25)
3 已知的等差中项为且,求的最小值。(参考答案:5)
4 设,若是与的等比中项,求的最小值。(参考答案:)
5 若,求的最小值。(参考答案:)
第二篇:高中数学第十章-排列、组合和二项式定理
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