28.已知复数z1=(a+i)2{z}_{1}={(a+i)}^{2}z1=(a+i)2,z2=4−3iz_{2}=4-3iz2=4−3i,其中a∈Ra\in Ra∈R.(1)若z1=iz2z_{1}=iz_{2}z1=iz2,求aaa的值;(2)若z1z2\frac{{z}_{1}}{{z}_{2}}z2z1是纯虚数,求aaa的值.