题目列表(包括答案和解析)
6.(福建3)设是等差数列,若,则数列前8项和为( C )
A.128 B.80 C.64 D.56
5.(全国Ⅰ7)已知等比数列满足,则( A )
A.64 B.81 C.128 D.243
4.(江西5)在数列中,, ,则 ( A )
A. B. C. D.
3.(宁夏8)设等比数列的公比q=2,前n项和为Sn,则=( C )
A. B. C. D.
2.(广东4)记等差数列{an}的前n项和为Sn,若S1=4,S4=20,则该数列的公差d= ( B )
A.7 B.6 C.3 D.2
1.(北京7).已知等差数列中,,,若,则数列的前5项和等于( C )
A.30 B.45 C.90 D.186
2.(江苏18)(16分)
设平面直角坐标系xoy中,设二次函数的图像与两坐标轴有三个交点,经过这三个交点的圆记为C。求:
(1)求实数b的取值范围
(2)求圆C的方程
(3)问圆C是否经过某定点(其坐标与b无关)?请证明你的结论。
[解析]:本小题考查二次函数图像和性质、圆的方程的求法。
(1)令x=0,得抛物线于y轴的交点是(0,b)
令f(x)=0,得x2+2x+b=0,由题意b≠0且△>0,解得b<1且b≠0
(2)设所求圆的一般方程为x2+ y2+Dx+Ey+F=0
令y=0,得x2+Dx+F=0,这与x2+2x+b=0是同一个方程,故D=2,F=b
令x=0,得y2+ Ey+b=0,此方程有一个根为b,代入得E=-b-1
所以圆C的方程为x2+ y2+2x -(b+1)y+b=0
(3)圆C必过定点(0,1),(-2,1)
证明如下:将(0,1)代入圆C的方程,得左边= 02+ 12+2×0-(b+1)×1+b=0,右边=0
所以圆C必过定点(0,1);
同理可证圆C必过定点(-2,1)。
1.(宁夏20)(本小题满分12分)
已知,直线:和圆:.
(Ⅰ)求直线斜率的取值范围;
(Ⅱ)直线能否将圆分割成弧长的比值为的两段圆弧?为什么?
解:(Ⅰ)直线的方程可化为,
直线的斜率,····························································································· 2分
因为,
所以,当且仅当时等号成立.
所以,斜率的取值范围是.·········································································· 5分
(Ⅱ)不能.················································································································· 6分
由(Ⅰ)知的方程为
,其中.
圆的圆心为,半径.
圆心到直线的距离
.·············································································································· 9分
由,得,即.从而,若与圆相交,则圆截直线所得的弦所对的圆心角小于.
所以不能将圆分割成弧长的比值为的两段弧. 12分
12.(湖北15).圆的圆心坐标为 (3,-2),和圆C关于直线对称的圆C′的普通方程是 . (x+2)2+(y-3)2=16
11.
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