"10 different Chinese books and 2 different mathematics books, what's the probability of taking out 2 mathematics books at will?" Why is the total number of "basic events" 12 × 11 △ 2 = 66

Because taking ab out is the same as taking Ba out, which is a case
So it's 12 × 11 / 2 = 66
P=1-（10*9/2)/66=21/66=7/22
In fact, in this problem, if you take the basic event as 12 × 11 = 132, the result is the same
p=1-(10*9/132)=7/22

10 different Chinese books, 2 different mathematics books, from which arbitrarily take 2, can take out the probability of mathematics books
Here's what I did: there are 132 possibilities in 12 * 11, and there are 22 possibilities in 2 * 11 out of any two books, but the answer is wrong,

It may be (12 * 11) / (2 * 1) instead of 12 * 11, because AB and Ba are the same
It could be 66 in all
The probability of not getting a math book is (10 * 9) / (2 * 1) = 45
So the probability of getting a math book is 1-45 / 66 = 7 / 22

First simplify, then evaluate: (2a-b) 2-b2, where a = - 2, B = 3

The original formula = 4a2-4ab + b2-b2 & nbsp; = 4a2-4ab. Substituting a = - 2, B = 3 into the above formula, the above formula = 4 × (- 2) 2-4 × (- 2) × 3 = 16 + 24 = 40

In the isosceles triangle ABC, ab = AC, be bisector angle ABC, CE bisector angle ACB, DF / / BC, point E is on DF, if AB = 2, find the perimeter of triangle ADF

Because DF ‖ BC be and CE are triangle bisectors
Therefore, Deb = EBC = Abe
Similarly, FEC = ECB = ace
So △ DBE and △ FEC are isosceles triangles
So de = DB Fe = FC
C（ADF）=AD+AF+DF=AD+AF+DB+FC=AB+AC=4

How many meters is one foot six

1 Zhang = 3.333 M
1 m = 0.3 Zhang
1.6 Zhang = 1.6 × 3.33 = 5.33

A rectangular playground with a length of 40 m and a width of 30 m is required to double its area, but the shape of the playground is not changed. How many meters should the length and width be increased
Accurate to 0.1M

If the length is x and the width is y, then x * y = 30 * 40 * 2 = 2400 (1), X: y = 40:30 = 4:3. If x = 4 / 3Y is substituted into (1), then 4 / 3Y square = 2400. After calculation, y = 30 root sign 2 = 42.3m, x = 40 root sign 2 = 56.4m, that is, 16.4m and 12.3m are increased

If a triangle has two internal angles whose sum of degrees equals 90 degrees, then the triangle must be a () triangle

Right triangle

If x < y, compare the size of 2X-4 and 2y-4, and explain the reason

Curious Xiao Ming wants to know how high the flagpole is. He measures the shadow of the flagpole by 10 meters. At the same time, he measures the shadow of a 6-meter-long tree nearby by 3 meters and 10 meters

Gold of Henan International

Ask questions about the part in brackets
Kitty's father wants (all the) hammers
（）（）（）kitty's father want?

how many hammers does
It should be four brackets

"10 different Chinese books and 2 different mathematics books, what's the probability of taking out 2 mathematics books at will?" Why is the total number of "basic events" 12 × 11 △ 2 = 66