11. N的阶乘公式:
N!=1*2*3*....(N-2)*(N-1)*N
且规定0!=1
例如
8!=1*2*3*4*5*6*7*8
12. 熟悉一下根号2、3、5的值
sqrt(2)=1.414
sqrt(3)=1.732
sqrt(5)=2.236
13. ...2/3 as many A as B: A=2/3*B
...twice as many... A as B: A=2*B
14. a if only b: b->a
