Definition of variance? How does a group of data multiply or divide by a number change its mean and variance? Add or subtract a number Master help it, dead teacher did not say let us do

Definition of variance? How does a group of data multiply or divide by a number change its mean and variance? Add or subtract a number Master help it, dead teacher did not say let us do

Variance is the average of the square of the difference between each data and the average
Suppose that the original average of this set of data is x and the variance is y
When it is multiplied or divided by a number a at the same time, the average is x multiplied or divided by a, and the variance is y multiplied or divided by the square of A
When it adds or subtracts a number B at the same time, the mean is equal to x plus or minus B. the variance is constant
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There is a sequence of numbers. The first number is 105 and the second number is 85. Starting from the third number, each number is the average of the first two numbers
Then the integral part of the 19th number is ()

A1 = 105, A2 = 85an = [a (n-1) + a (n-2)] / 2an-a (n-1) = - 1 / 2 * [a (n-1) - A (n-2)] so {B (n-1) = an-a (n-1)} is a proportional sequence a (n + 1) - an = - 20 * (- 1 / 2) ^ (n-1) with the first term of a2-a1 = - 20 and the common ratio of q = - 1 / 2, that is, a2-a1 = - 20 * q ^ 0a3-a2 = - 20q ^ 1... An-a (n-1) = - 20q ^ (n-1)

Sequence lg1000, LG (1000 · cos60 °), LG (1000 · cos260 °) lg（1000•cosn-160°），… Before______ Is the sum of items the largest?

The general term an = 3 + (n-1) LG12 of the sequence is monotonically decreasing, and the tolerance D is less than 0. Because when an = 3 + (n-1) LG12 is less than 0, n ≤ 10, so when n ≤ 10, an < 0, we can know that the first 10 items of the sequence are all positive, and negative from the 11th, so we can know that the sum of the first 10 items of the sequence is the largest. So the answer is: 10