521×27+21921×57+(1−27)．

521×27+21921×57+(1−27)．

521×27+21921×57+（1-27），=521×27+21921×57+57，=521×27+（21921+1）×57，=521×27+8221×57，=221×57+8221×57，=（221+8221）×57，=4×57，=207．

1/3+1/5+1/7+.+1/19+1/21=?
A simple method,

What are you doing?
There is no simple algorithm for this sequence
1 + 1 / 2 + 1 / 3 +... + 1 / N, this is a harmonic sequence
1+1/2+1/3+...+1/n≈ln(n)+C,C=0.5772
This is also an approximate formula when n is very large
So, 1 / 3 + 1 / 5 +... + 1 / 21 really has no simple algorithm
It can only be calculated by hardware or by calculator or by software
1/3+1/5+...+1/21≈1.1809

In △ ABC, we know that a = 20, B = 29, C = 21, and we can solve triangles

A = 20 A ^ 2 = 20x20 = 400 B = 29 B ^ 2 = 29X29 = 841c = 21 C ^ 2 = 21x21 = 441, that is: B ^ 2 = a ^ 2 + C ^ 2, satisfy the Pythagorean theorem, the triangle is a right triangle, so: B = 90 ° from the sine theorem: A / Sina = B / SINB Sina = A / b = 20 / 29 = 0.689a = 43 ° C = 180 ° - 43 ° - 90 ° = 47 °

Known X / 17

4/5=0.8
17*0.8=13.6
X is 13
Y is 14
13/17

Given that natural numbers x and y satisfy 9x + 8y = 7, then the value of X + y is______ ．

∵ natural numbers x and y satisfy 9x + 8y = 7, ∵ 9x = 3, 8y = 4, the solution is x = 3, y = 2, ∵ x + y = 3 + 2 = 5

In 9 / X and 49 / y, X and y are natural numbers, and 9 / X is a true fraction, and 9 / x equals 49 / y multiplied by 7 / 21, and x plus y equals several

There are some loopholes in Theodore's answer
9/x=(49/y) * (7/21)
Transfer: 27 / x = 49 / Y
Then: 27y = 49x
Since the greatest common factor of 27 and 49 is 1, it can only be y = 49n, x = 27N, where n is any positive integer
In this case, 9 / x = 9 / (27N) = 1 / (3n). Since n is a positive integer, it satisfies the condition that 9 / X is a true fraction
So x + y = 76n, n is any natural number

If 2A + 3b-1 < 3A + 2B, then A-B ()

Simple
Because it's a rectangle, it's two longer, with a circumference of
2a+3b+A+B-2*(1/2(A-B))=2a+2b+2B
Positive solution

What are the prime numbers in 1,2,35200,49,39,82,17,91,77,46,12,48,41

1,2,17,41

Calculate 83 degrees 46 points + 52 degrees 39'16 ″

1°=60'=3600''
83°46'+52°+39'16''
=135°85'67''
After simplification = 136 ° 26'7

The following IP addresses are valid () a.172.17.1.4 b.202.199.175.60 c.230.22.1.1 d.270.2.140.52
Can you explain in detail why?

The answer D is obviously wrong. The decimal system of each dot of IP address can't be greater than 255. Although the answer c starts with 230, it's a reserved class D address and can't be used by personal computers
If you can choose two answers, a and B are both correct, and both are legal IP addresses. But if you can only choose one, then the answer is B, because a is a private address and cannot be transmitted in Wan. B's address is available in LAN and WAN