为什么当x∈(0,π)时,cosx<1
\u5f53x\u8d8b\u4e8e0\u65f6(1-cosx)^2\u662fx^2\u7684\uff08\uff09\u6cf0\u52d2\u5c55\u5f00cosx=1-x^2/2
(1-cosx)^2=x^4/2\u662fx^2\u7684\u9ad8\u9636\u65e0\u7a77\u5c0f
\u4e5f\u5c31\u662f(x^4/2)/x^2=x^2/2=0
\u7b49\u4ef7\u65e0\u7a77\u5c0f
它什么时候都不大于1当x=2kπ时cosx=1，而定义域里没有，所以小于1
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