“若A则B”的意思是说:A是B的充分条件,但A并不一定是B的必要条件。对这一基本概念的理解也经常运用到解选择题和编拟选择题上,下面看几个例子。例1.θ为三角形的一个内角,若函数y=cosθ·x~2-4sinθ·x+6对于任意实数x都有y>0,则cosθ (A)大于0;(B)等于0; (C)小于0;(D)不能确定正负号。我们用排除法可得正确答案是cosA> “If A then B” means that A is a sufficient condition for B, but A is not necessarily a necessary condition for B. The understanding of this basic concept is also often applied to multiple-choice questions and the preparation of multiple-choice questions. Here are a few examples. Example 1. θ is an interior angle of a triangle. If the function y=cosθ·x~2-4sinθ·x+6 has y>0 for any real number x, then cosθ (A) is greater than 0; (B) is equal to 0; C) Less than 0; (D) No sign can be determined. The correct answer we can get with the exclusion method is cosA>