定义a⊙b=2a-b,求[(x+y)⊙(x-y)]⊙(x-2y)的值,其中x=3,y=5/2.

x+y=3+5/2=11/2
x-y=3-5/2=1/2
∴(x+y)⊙(x-y)
=2×11/2-1/2
=11-1/2
=21/2
x-2y
=3-2×5/2
=3-5
=-2
∴[(x+y)⊙(x-y)]⊙(x-2y)
=2×21/2-（-2）
=21+2
=23