cos4等于什么？是2倍的cos2吗？ 4分之二倍的cos2x加一等于多少？
1+cos4\u662f\u600e\u4e48\u7b49\u4e8e2cos²2\u7684\uff1f\uff1f\uff1f\uff1fcos4=cos\u65b92-sin\u65b92\u30021+cos4=cos\u65b92+1-sin\u65b92=2cos\u65b92
\u539f\u5f0f=(cosx)^2.
不是，1=180度/派，所以4=720度/派，cos4=2(cos2)^2+1不是
cos4=cos²2-sin²2=2cos²2-1=1-2sin²2
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