cos^2x＜sin^2x
cos^2x-sin^2x＜0
cos(2x）＜0
(2k+1)π-π/2 ＜ 2x ＜ (2k+1)π+π/2
kπ+π/4 ＜ x ＜ kπ+3π/4,其中k∈Z