对于有理数a、b,定义a⊙b=3a+2b,则[(x+y)⊙(x-y)]⊙3x化简后得(  ) A.0 B.5x C.21x+3y D.9x+6y

问题描述:

对于有理数a、b,定义a⊙b=3a+2b,则[(x+y)⊙(x-y)]⊙3x化简后得(  )
A. 0
B. 5x
C. 21x+3y
D. 9x+6y

∵a⊙b=3a+2b,
∴[(x+y)⊙(x-y)]⊙3x
=[3(x+y)+2(x-y)]⊙3x
=(3x+3y+2x-2y)⊙3x
=(5x+y)⊙3x
=3(5x+y)+2×3x
=15x+3y+6x
=21x+3y.
故选C.