1974 AMC 12 Question 6
AMC12 · 1974 AMC 12
For positive real numbers $x$ and $y$ define $x*y=\frac{x\cdot y}{x+y};$ then
“$*$” is commutative but not associative
“$*$” is associative but not commutative
“$*$” is neither commutative nor associative
“$*$” is commutative and associative
none of these