cosx单调区间为：[2kpi,2kpi+pi]递减,[2kpi-pi,2kpi]递增
令
1/2*x-pi/6=2kpi,解得x=4kpi+pi/3
1/2*x-pi/6=2kpi+pi,解得x=4kpi+7pi/3
1/2*x-pi/6=2kpi-pi,解得x=4kpi-5pi/3
所以递减区间为[4kpi+pi/3,4kpi+7pi/3]
递增区间为[4kpi-5pi/3,4kpi+pi/3]