诱导公式.
1.已知cos31°=a,则sin239°tan149°=_____
2.求三角函数值(1)cos(-17π/4)(2)sin26π/3(3)cos1650°(4)sin1740°
3.[sin(180°-405°)sin(270°-765°)]/[sin(90°+45°)tan(270°+45°)]=______
4.当角α,β满足的条件为_____,有sinα=sinβ