∵f(a/2 - π/6)=2∴tan(a-π/3+π/3)=2tana=2∵a∈(0,π/2)∴cosa=√[1/(1+tan²a)]=√5/5sina=√(1-cos²a)=2/5·√5∴cos(a+π/4)=cosacosπ/4-sinasinπ/4=√5/5×√2/2-2√5/5×√2/2=-√10/10...