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4. A=sin( ωt +φ )
A振幅=10 , ω =2π/T =20 π , φ<0,表示落後
由上而下
10sin(20π t) , 10sin[20π (t+0.25) ) -0.5π] , 10sin[20π (t+0.5) ) -π]

5.
Re {5je^(j2θπt) } =Re { 5j (cos2θπ t +j sin2θπ t) }
= -5 sin2θπ t = 5 cos (2θπ t + π/2)

Re {-5je^(j2θπt) } =Re { -5j (cos2θπ t +j sin2θπ t) }
= 5 sin2θπ t = 5 cos (2θπ t + 3π/2)

P.S. jˊ2 = -1 , cos (θ+π/2) = -sinθ ,cos (θ+3π/2) = sinθ



ref .

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