tana=sina/cosa=√2sina=√2*cosa(sina)^2=2(cosa)^2因为(sina)^2+(cosa)^2=1所以(cosa)^2=1/32(sina)^2-sinacosa+(cosa)^2=2*2(cosa)^2-√2*cosa*cosa+(cosa)^2=(4-√2+1)(cosa)^2=(5-√2)*1/3=(5-√2)/3