1. in the xy-plane, is point (2,-3) on line L?
1) POINT (-2,3) is on L.
2) L is not perpendicular to THE x-axis.
从2中得知, L 平行于X 轴(是不是?), 有没有相交的可能?
2. if x is an integer, is (54+27)/x an integer?
1) 6<= x <= 81
2) x is a multiple of 3.
x is a multiple of 3 和 6<= x <= 81 的条件, 就可推出. 为什么答案是E?
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