sin40°=m,(改题了),则m^2=(sin40°)^2=(1-cos80°)/2=(1-sin10°)/2,所以sin10°=1-2m^2,cos20°=1-2（sin10°）^2=1-2(1-2m^2)^2=1-2(1-4m^2+4m^4)=-1+8m^2-8m^4.
