1, 2, negative 3, 1, negative 4, 5, 6, negative 7 -3，-4，-7，_______ (2014)

1, 2, negative 3, 1, negative 4, 5, 6, negative 7 -3，-4，-7，_______ (2014)

The first two are positive and the last two are negative
2014 / 4 = 503 more than 2, so it is positive

There are several ways to choose three numbers randomly from 1, 2, 3, 4, 5, 6, 7, 8 and 9 so that their sum is odd
The principle of addition is required
They were divided into two groups
Group 1: 1, 3, 5, 7, 9
Group 2: 2, 4, 6, 8
Choose one in group one and two in group two: C, 1,5 × C, 2,4 = 5 × 6 = 30
Choose three in group one and none in group two: C, 3,5 = 10
So there are 40 choices
Choose two in group 2: how do you calculate 5 and 6 in group 2?
No choice in group 2: what's the result of middle 10?

There are two groups: group 1: 1, 3, 5, 7, 9, group 2: 2, 4, 6, 8, choose one in group 1, choose two in group 2: C, 1,5 × C, 2,4 = 5 × 6 = 30, choose three in group 1, not choose three in group 2: C, 3,5 = 10, so there are 40 kinds of selection methods. Have you ever learned permutation and combination? "C, 1,5" means choose one out of five, there are five kinds

Number 1.2.3.4.5.6.7.8.9, 9 in total,
Four different numbers are selected to form two double digits, so that the sum of the two double digits is a multiple of 3. How many different double digits are there?

18,81,27,72,36,63,45,54,
12,21,15,51,24,42,27,72
39,93,48,84,57,75,69,96
87,78