(1)a·b=cos23°cos68°+ cos67°cos22°
=cos23°cos68°+ sin23°sin68°
=cos(68°-23°）
=cos45°
=√2/2
|a|=1,|b|=1
(2)|u|²=a²+t²b²+2ta·b
=t²+√2t+1
=(t+√2/2)²+1/2
当t=-√2/2时,|u|²有最小值1/2,|u|有最小值√2/2