∵cos(x+20)=1/3,x∈(0,90)
∴sin(x+20)=2√2/3
cos(x+80)=cos(x+20+60)=cos(x+20)cos60-sin(x+20)sin60
=(1/3)*(1/2)-(2√2/3)*(√3/2)
=(1-2√6)/6