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； 2、由
推导两角和的余弦公式
. 3、已知△ABC的面积
，且
，求
.
参考答案：
参考解析：
刷刷题刷刷变学霸
举一反三
【单选题】Task 1 Directions: After reading the following passage, you will find 5 questions or unfinished statements, numbered 36 through 40. For each question or statement there are 4 choices marked A, B, C, a...
A.
in his mind
B.
in his laboratory
C.
with microscope
D.
with telescopes