Sin(x+π/2)=cosx=1/3,则sinx=土2√2/3,cos(7π/12)=cos(π/3+π/4)=(√2-√6）/4,sin(7π/12)=(√2+√6）/4,∴cos(x+7π/12)=cosxcos(7π/12)-sinxsin(7π/12)=（√2-√6）/12干（4+4√2）/12=（-4-3√2-√6）/12,...