1) (y-2)/(x-3)=1/5；∴线段PQ的表达式为y=1/5x+7/5,-2≤x≤3.带入直线 ,x=(17/5)/(a+1/5)=17/(5a+1).
-2≤17/(5a+1)≤3,5a+1≠0；解不等式得a≤-19/10,或 a≥14/15.
2) 应该是 -4才对.
3) y=ax^2+bx+c,c=1; ak^2+bk+1=a(k+2)^2-b(k+2)+1=ak^2+4ak+4a-bk-2b+1,(2b-4a)(k+1)=0;
b=2a; 令x1,x2为y的两个零点,x1+x2=-2a; x1x2=1,(x1-x2)^2=x1^2+x2^2-2x1x2=(x1+x2)^2-4x1x2=4a^2-4=8,a=±1；∴y=x^2+2x+1或y=-x^2-2x+1