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This issue has been reported at https://ask.sagemath.org/question/33623/bug-in-cos-function-in-version-72/?answer=33624#post-id-33624 and https://trac.sagemath.org/ticket/20752 and fixed in Sage 7.3.beta4, see https://trac.sagemath.org/ticket/20742 So, if you are running a Sage version, less than 7.4, let me just suggest to upgrade.