1.cos2x=-1/9
2cos^2x-1=1/9 cos^2x=5/9
1-2sin^2x=1/9 sin^2x=4/9
tan平方x乘sin平方x=sin^2x*sin^2x/cos^2x=(4/9)*(4/9)/(5/9)=16/45
2.tan10度tan20度+根号3（tan10度+tan20度）
=(sin10°sin20°)/(cos10°cos20°)+√3(sin10°/cos10°+sin20°/cos20°)
=[sin10°sin20°+√3(sin(10°+20°)]/(cos10°cos20°)
=[-(1/2)cos30°+(1/2)cos10°+√3/2]/(cos10°cos20°)
=(1/2)(cos10°+√3/2)/(cos10°cos20°)
=(1/2)(cos10°+cos30°)/(cos10°cos20°)
=(cos10°cos20°)/(cos10°cos20°)
=0
我写详细些,不然会少好多步骤,