cosx=根号2/2
x=π/4＋2kπ或者x=-π/4＋2kπ
因为x大于-4π小于4π
所以x=π/4－4π x=π/4－2π x=π/4 x=π/4＋2π
x=-π/4－2π x=-π/4 x=-π/4＋2π x=-π/4＋4π