|cosa|＜|sina|
cos²a＜sin²a
1-sin²a＜sin²a
1-2sin²a＜0
cos(2a)＜0
∴2a∈(π/2+2kπ,3π/2+2kπ),k∈Z
∴a∈(π/4+kπ,3π/4+kπ),k∈Z
祝愉快