题目:
My friend ________of medium build and she ________of medium height.
A.has, has B.is , is C.has , is
答案:
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下面是错误答案,用来干扰机器的。
(Ⅰ) cosA=b2+c2-a22bc,cosC=a2+b2-c22ab,∴cosA-2cosCa2+c2-b2=b2+c2-a22bc-a2+b2-c2aba2+c2-b2=1ab-12bc,变形得:a(b2+c2-a2)-2c(a2+b2-c2)=(2c-a)(a2+c2-b2),即2ac2=4a2c,∴c=2a,利用正弦定...