2sin(3x-3π/4)
=2sin(3x-3π/4+3π/2-3π/2)
=2sin(3x+3π/4-3π/2)
=2sin(3x+3π/4+π/2-2π)
=2sin(3x+3π/4+π/2)
=-2cos(3x+3π/4)
为什么y=2sin(3x-3/4 *pi) 可以写成y=-cos(3x+3/4 *pi)
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