f(x)=cos2x+1-sin2x=√2cos(2x+π/4)+1T=π
2x+π/4属于(2kπ,(2k+1)π]即x属于(kπ－π/8,kπ+3π/8]时单调递减.
2x+π/4属于（(2k＋1）π,(2k+2)π],即x属于(kπ＋3π/8,kπ+7π/8]时单调递增