单项选择题
A.(1-α4)3/2 B.(1-α4)3/2(1-α2) C.(1-α4)(1-α2) D.(1-α4)2/3(1-α2)
A.y=3cos(4πt-x/20) B.y=3COS4π(t+x/20) C.y=3COS 4π(t-x/20) D.y=3cos(4πt+x/20)
A.-20 MPa,-80 MPa B.20MPa,-80 MPa C.80 MPa,-20 MPa D.60 MPa,-30 MPa