13. 观察下列等式:
(1 1) 2 1+ = ´
( $2 + 1 ) ( 2 + 2 ) = 2 ^ { 2 } \times 1 \times 3$
$$ ( 3 + 1 ) ( 3 + 2 ) ( 3 + 3 ) = 2 ^ { 3 } \times 1 \times 3 \times 5 $$
照此规律, 第n个等式可为
Answer key
$( n + 1 ) ( n + 2 ) ( n + 3 ) \cdots ( n + n ) = 2 ^ { n } \cdot 1 \cdot 3 \cdot 5 \cdots ( 2 n - 1 )$