楼上的算错了,纠正如下：
y1+y2=5cos(6x-900t)+5cos(6x-900t-2)
=5*2*cos{[(6x-900t)+(6x-900t-2)]/2}*cos{[(6x-900t)-(6x-900t-2)]/2}
=10*cos1*cos(6x-900t-1)