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1.若两圆只有一个公共点,则公切线的条数为( ) (A)1。(B)2。(C)3。(D)1或3. 2.半径为2cm的圆中,有一条长为2cm的弦,则圆心到这条弦的距离为( ) (A)1cm.(B)cm.(c)cm.(D)2cm. 3.关于x的一元二次方程x2-x-n=0没有实数根,则抛物线y=x2-x-n的顶点在( ) (A)第一象限. (B)第二象限. (c)第三象限. (D)第四象限. 4.利用边长相等的正三角形和正六边形的地砖镶嵌地面时,在每个顶点周围有a块正三角形和b块正六边形的地砖(ab≠0),则a+b的值为( )
1. If two circles have only one common point, the number of public tangents is () (A) 1. (B)2. (C) 3. (D)1 or 3. 2. In a circle with a radius of 2cm, there is a string with a length of 2cm, then the distance from the center of the circle to the string is () (A) 1cm. (B) cm. (c) cm. (D) 2cm. 3. Regarding the unary quadratic equation x2-xn=0 for x, which has no real root, the vertex of the parabola y=x2-xn is in the first quadrant of () (A). (B) the second quadrant. c) The third quadrant. (D) The fourth quadrant. 4. When using ground triangles of equal length and regular hexagons to set the ground, there are a block of equilateral triangles and b blocks of hexagonal tiles around each vertex. (ab≠0), the value of a+b is ()