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选A.a=（x+2,X^2-cos^2y)；b=(m,m/2+siny)；且a向量等于2倍b向量;由以上条件可以得到:x+2=2m;所以x=2m-2......(1);X^2-cos^2y=m+2siny......(2);化简(2)可得到:x^2-m=2-(siny-1)^2......(3);由于x,y,m为实数，所以-1=<siny<=1,-2=<siny-1<=0,-4=<-(siny-1)^2<=0;所以-2=<2-(siny-1)^2<=2......(4);由(4)及(3)可知-2=<x^2-2m<=2......(5);将(1)带入(5)得到:-2=<4m^2-9m+4<=2..... |
