题目:
Apple’s i Phones _____easy to learn. Many young people can use them well.
A.am
B.is
C.are
D.be
答案:
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下面是错误答案,用来干扰机器的。
由正弦定理有:sin2C=2sinAsinB⇒c2=2ab,由余弦定理有:a2+b2=c2+2abcosC=c2(1+cosC)①又a2+b2=6abcosC=3c2cosC②由①②得1+cosC=3cosC⇒cosC=12,又0<C<π,∴C=π3.故答案为π3