How to use oxygen volume fraction to calculate oxygen mass fraction (oxygen density 1.42g/l, air 1.29g/l, oxygen volume fraction 21%)

How to use oxygen volume fraction to calculate oxygen mass fraction (oxygen density 1.42g/l, air 1.29g/l, oxygen volume fraction 21%)

If a certain volume of air is x L, then:
The total air mass is 1.29 x G
The mass of oxygen is 1.42 × 0.21 x G
The mass fraction of oxygen is 1.42 × 0.21 / 1.29 × 100% = 23%

For two natural numbers a and B (both a and B are greater than 0), a new operation "#" is defined
Then (2 × 6) × 16 = ()

114/7

In the cycle of decimal 0 and abcabc, it is known that the sum of the first 1000 digits on the right side of the decimal point is 4664, and there are two equal numbers in a, B and C. what is the cycle decimal?
Write the answer and say why

There are 3 digits in each cycle section of this cycle decimal, and 1000 / 3 = 333 (cycle sections) in 1000 digits
4664 / 333 = 14 (sum of numbers on each bit of each cycle section). 2 (that is, the remaining one digit, that is, the first digit of cycle section is 2)
Since two digits in each cycle section are equal, so (14-2) / 2 = 6
The three numbers of each cycle section are 2, 6 and 6

How far can a light year travel?

Light year is a unit of length, that is, the distance light travels in a year
1 light year is s = V * t = (3 * 10 ^ 8) * (365 * 24 * 3600) = 9.5 * 10 ^ 15m = 9.5 * 10 ^ 12km

Zu Chongzhi, an ancient mathematician in China, had calculated the approximate value of PI between 3.1415926 and 3.1415927 in the 5th century,
At this time, the approximate value of PI n has been accurate to ()

Millionths

Find the center of form, from the plane curve y = x square and y = 1 besieged plane area

Triangle, area 1 / 2

Given log2 (5) = a, find LG2 * lg5

log2 （5） = lg5/lg2=a
Because lg5 = 1-lg2 = alg2
lg2=1/(1+a)
lg2*lg5=alg2*lg2=a / [(1+a)^2]

If a △ B = 5, then a must be divisible by B______ (judge right or wrong)

If both a and B are integers, it can be said that a can be divisible by B. If one or two of a and B are not integers, it can not be said that a can be divisible by B

Given the ellipse 16 / x2 + 4 / y2 = 1, the equation for finding the straight line of the chord with P (2. - 1) as the center

Let the linear equation be y = KX + B, intersect with ellipse X & # 178 / / 16 + Y & # 178 / / 4 = 1 at two points a (X & # 8321;, Y & # 8321;) and B (X & # 8322;, Y & # 8322;)
y₁=kx₁+b ①
y₂=kx₂+b ②
x₁²/16+y₁²/4=1 ③
x₂²/16+y₂²/4=1 ④
And P (2, - 1) is the midpoint of the string AB, so (X &; + X &;) / 2 = 2, X &; + X &; = 4, (Y &; + Y &;) / 2 = - 1, Y &; + Y &; = - 2;
①-② y₁-y₂=k(x₁-x₂）,k=（y₁-y₂）/(x₁-x₂) ⑤
③ - ④ (X &; + X &;) (X &; - X &;) / 16 + (Y &; + Y &;) (Y &; - Y &;) / 4 = 0, bring in X &; + X &; = 4, Y &; + Y &; = - 2,
In this paper, we obtain X &; - X &; = 2 (Y &; - Y &;), from 5, so k = 1 / 2;
① (2) y &; + Y &; = K (X &; + X &;) + 2B, bring in X &; + X &; = 4, Y &; + Y &; = - 2, k = 1 / 2, get b = - 2;
In conclusion, the linear equation is y = (1 / 2x) - 2

In natural numbers 1-20, the sum of numbers that are not even numbers has______ Prime numbers that are not odd have______ ．

In the natural number 1-20, there are 9 and 15 which are not even numbers, and 2 which are not odd numbers