tan(π-a)=3 =>-tanα=3 =>tanα=-3
[2sin(π/2-α）-3sin（π+α）]/[4cos(-α）+sin（2π-α）] 由诱导公式知
=(cosα+3sinα)/(cosα+sinα) 同除cosα
=(2+3tanα)/(4+tanα)
=-7