2011-11-22
1. 有理数的乘法
- 两数相乘,同号得正,异号得负,并把绝对值相乘
- 多个数相乘,负数的个数为偶数时,得正;负数的个数为奇数时,得负
- 任何数同0相乘,都得0
- 乘积是1的两个数互为倒数
5 × 3 = 15
5 × (-3) = -15
-5 × 3 = -15
-5 × (-3) = 15
2 × (-3) × 4 × (-5) × (-6) = -720
2. 有理数的除法
- 除以一个不等于0的数,等于乘上这个数的倒数
3. 乘法交换律、结合律、分配律
乘法交换律:ab = ba
乘法结合律:(ab)c = a(bc)
乘法分配律:a (b + c) = ab + ac
练习:
1. a、b、c为非零有理数,并且a+b+c = 0,求的值
2. 找出 |a|b > ab, |a|b > a|b|, a/b > ab, a/b > -|a|b 这些式子成立时,a、b的取值范围.
SAT:
During last year’s baseball season, a certain player has 625 at bats and gets a hit 32% of the time. This season the player increased the at bats by 12%, and got a hit 34% of the time. What is the percent increase in the number of hits?
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