1998 年

Find the mean deviation of the five numbers $x-2$, $x-1$, $x$, $x+1$ and $x+2$.

The circle $x^2+y^2-2x-7y-8=0$ intersects the $x$-axis at $A$ and $B$. Find the length of $AB$.

The equations of two circles are $x^2+y^2+ax-by=0$ and $x^2+y^2-ax+by=0$. Which of the following must be true? I. The two circles have the same centre. II. The two circles have equal radii. III. The line joining the centres of the two circles passes through the origin.

$A(7,14)$ and $B(1,2)$ are two points. $C$ is a point on $AB$ produced such that $AB:BC=2:1$. Find the coordinates of $C$.