高中數學麻煩~~~~?

6. i^86+i^87+i^88+.........+i^2000
= i^86+i^87+i^88+0
= - 1 - i + 1
= - i
=a+bi
a=0, b= - 1
Ans: a-b=0-(-1)=1

7. x^2+(a+i)x-3(1+i)=0
[x+(1+i)](x-3)=0
x^2+(-2+i)-3(1+i)=0
Ans: a= - 2

8.
f(0)f(1)<0 ---> (-5)(2k+k-1-5)<0 ---> k>2
f(1)f(-2)<0 ---> (2k-k+1-5)(8k-2k+2-5)<0 ---->(k-4)(2k-1)<0 ----> 1/2<k<4
Ans: 2<k<4