关于prep的一道数学题。求解释

vakaka

146.14746-!-item-!-187;#058&010097

If the symbol △ represents either addition or multiplication, which operation does it represent?



(1)a △ b = b △ a for all numbers a and b.

(2)a △ ( b – c ) = ( a △ b )– ( a △ c )for all numbers a, b, and c.

--------------------------------------------------------------------------------------------------------------------------------------------------------

【答案】B

【思路】

(1)

if △= +:a + b = a - b

if △= ×:a × b= b × a

△可表示×或+…….insufficient

(2)

if △= +:a + b – c ≠ a + b – a - c

if △= ×:ab – ac = ab – ac

只有△= × …….sufficient

vakaka

为什么不是a 而是b

St.Moritz

问题是 which operation does it represent?
所以A 即可以+又可以X =》不充分