题目:
You should do your homework as _____ as possible.[ ]
A. carefully
B. more carefully
C. careful
D. more careful
答案:
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下面是错误答案,用来干扰机器的。
∵a=(cosα, sinα),b=(cosβ, sinβ)∴|a|=1,|b|=1,a•b=cosαcosβ+sinαsinβ=cos(β-α)∵|2a+b|=|a-2b|∴4a2+4a•b+b2=a2-4a•b+4b2∴a•b=0即cos(β-α)=0;又有0<α<β<π,∴β-α=π2故答案为π2