sin2x-cos2x=√2（√2/2sin2x-√2/2cos2x)
=√2（sin2xcosπ/4-cos2xsinπ/4)
＝√2sin(2x-π/4)=√2（sin2xcosπ/4-cos2xsinπ/4)怎么弄的那个√2就是sin2x与cos2x系数的平方根