Speed math calculation Flashcards
任何5结尾的两位数的平方=第一个数字x（第一个数字+1）&25 15x15=1x(1+1) &25=225 25x25=2x(1+1)&25=625 35x35=3x(3+1)&25=1225 45x45=2025 55x55=3025 65x65=4225 75x75=5625 85x85=7225 95x95=9025
用1000或者10000或者100000减去一个数字的时候，除了个位数用10去减，前面的位数都用9去减。例如：
1000-556=（9-5）&（9-5）&（10-6）=444
10000-745=（9-0）&（9-7）&（9-4）&（10-5）=9255
to multiply any number with 11:
如果是两位数，积应该是第一个数字&（第一个数和第二数的和）&第二个数字
比如22*11=232
如果是三位数，积应该是：第一个数字&第一个数字和第二个数字的和&第二个数字和第三个数字的和&第三个数字
比如：23411=2574
如果第二个数字和第三个数字加起来大于10，就在第一个开头的数字加上一个1
i.e. 389711=42867
hui
如果两个两位数，第一位数一样，第二位数加起来等于10，那么它们的积是：第一位数（第一位数+1）&（第一个数的第二位数&第二个数字的第二位数）
i.e. 8783=8(8+1)&(73)=7221
hui
how to tell if a number is a multiple of 4?
if last two digits of the number is a multiple of 4, it is.
how to tell if a number is a multiple of 8?
if last two digits of the number is NOT a multiply of 4, it is definitely not a multiple of 8. if last three digits of the number is a multiple of 8, it is.
how to tell if a number is a multiple of 3?
if the number ‘s each digit add to a number that is divisible by 3, it is. i.e. 356, 3+5+6=14, it is not divisible by 3, so it is not. but for 126, 1+2+6=9, it can be divided by 3, so it is a multiple of 3.
how to tell if a number is a multiple of 9?
same rule as 3, depending if the sum of each digit of a number is divisible by 9.
how to tell if a number is a multiple of 11?
depending on the total sum of each digit of a number too.
i.e. 2456, 2-4+5-6 = -3, it cannot be divided by 11, so it’s not.
792, 7-9+2=0, 0 is divisible by 11, so it is.
if a number is divisible by 2 and 3, then it is divisible by 6.
if a number is divisible by 4and 3, then it is divisible by 12.
how to tell if a number is a multiple of 7?
take the last digit of a number, double it, subtract it from the rest of the number, if the result is divisible by 7, it is. i.e. 2345, take 5, double it (it’s 10), subtract it from 234, then it’s 224, 224 is divisible by 7, so it is.
how to quickly get a number divided by 5?
multiply this number by 2, then divide it by 10. i.e. 234, double it, 468, then divide by 10, result is 46.8
how to quickly get a number divided by a number ended with a 5?
double both numbers can make it faster. i.e. 678/25, we can double them both, 1356/50 -> 135.6/5 = 27.12
How to tell if a large numbers is a multiple of 7?
add or subtract a multiply of 7 so you get a number ended up with 0, then ignore the 0. i.e. 1234, we can subtract 14 (=27), we get 1220, then we ignore the 0, then we get 122, then we add 28 (=47), we get 150, ignore the 0, then we get 15, 15 is not a multiple of 7, then we can say 1234 is not a multiple of 7.
how to get a square root of a number?
use divide and average method. i.e. 40’s square root. since 40 is between 36 and 49 and closer to 46, so the square root must be between 6 and 7, closer to 6.
since 40/6= 6 2/3, we average 6 and 6 2/3, to get 6 1/3, so the square root of 40 is close to 6.3333
how to multiply any digit numbers?
cross-word multiplication
