27.已知复数z1=a2−2ai(a∈R)z_{1}=a^{2}-2ai(a\in R)z1=a2−2ai(a∈R),复数z2z_{2}z2在复平面内对应的向量为OA→=(1,−2)\overrightarrow{OA}=(1,-2)OA=(1,−2).(1)若z1−z2z_{1}-z_{2}z1−z2为纯虚数,求aaa的值;(2)若z1i+z2z_{1}i+z_{2}z1i+z2在复平面内对应的点在第三象限,求aaa的取值范围.