问题
选择题
— Sorry, I got a bad cold yesterday.
— Every time you are absent, you ______ an excuse. [ ]
A. take up
B. give up
C. send up
D. make up
答案
答案：D
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— Sorry, I got a bad cold yesterday.
— Every time you are absent, you ______ an excuse. [ ]
A. take up
B. give up
C. send up
D. make up
答案：D
| 阅读下面材料： 根据两角和与差的正弦公式，有 sin（α+β）=sinαcosβ+cosαsinβ------① sin（α-β）=sinαcosβ-cosαsinβ------② 由①+②得sin（α+β）+sin（α-β）=2sinαcosβ------③ α+β=A，α-β=B 有α=
代入③得 sinA+cosB=2sin
（1）类比上述推理方法，根据两角和与差的余弦公式，证明：cosA-cosB=-2sin
（2）若△ABC的三个内角A，B，C满足cos2A+cox2C-cos2B=1，直接利用阅读材料及（1）中的结论试判断△ABC的形状． |