What is 1 + one third + one sixth + one tenth + one fifteenth + one twenty-one Come on

What is 1 + one third + one sixth + one tenth + one fifteenth + one twenty-one Come on

1 + 1 / 3 + 1 / 6 + 1 / 10 + 1 / 15 + 1 / 21
=2×(1/2+1/6+1/12+1/20+1/30+1/42）
=2×(1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7）
=2×(1/2+1/2-1/7）
=2×6/7
=12 / 7 (12 / 7)

How much is one third plus one sixth plus one tenth plus one fifteenth plus one twenty-one

One third plus one sixth plus one tenth plus one fifteenth plus one twenty-one
=(1 / 3 plus 1 / 6) plus (1 / 10 plus 1 / 15) plus 1 / 21
=1 / 2 + 1 / 6 + 1 / 21
=2 / 3 + 1 / 21
=5 out of 7

One third plus one sixth plus one tenth plus one fifteenth plus one twenty-one plus one twenty-eight

Answer: three quarters
Methods: the first two items were added, the middle two items were added, and the last two items were added

A simple method is used to calculate (1 / 102) x (1 / 12) x (1 / 103) x (1 / 13) X X (1 / 10 / 98) x (1 / 10 / 98) x (1 / 10 / 99) x (1 / 10 / 99)

Exchange items first
The original formula = (1-1 / 2) (1 + 1 / 2) (1-1 / 3) (1 + 1 / 3) (1-1 / 4) (1 + 1 / 4) × ×(1-1/99)(1+1/99)
I'll work it out in parentheses
=1/2 ×3/2×2/3 × 4/3×3/4 ×…… ×99/98×98/99 ×100/99
=1/2×100/99
=50/99

One plus one third plus one sixth plus one tenth plus one fifteenth plus one twenty-one and so on plus two thousand nine percent

The general formula a (n) = 1 / (1 + 2 + 3 +... + n) = 1 / N!
n!=n(1+n)/2
So a (n) = 2 / N (1 + n) = 2 (1 / n-1 / (n + 1))
S(n)=2((1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/n-1/(n+1)))
=2(1-1/(n+1))
Find n
a(n)=2/n(1+n)=1/2009
So n =?
After the solution, bring it to s (n) to find the answer
You are sure that the final result is 2009, but 2009 * 2 is not the factorial of any integer. The above is the answer when the general formula is n

Simplification, calculation
1. Simplification
(1) -(+7) (2) -[-(-3)] (3) -|+[-(-200]| (4) +[-(-1/70]
Ask for the answer, because I don't understand
2. Calculation
If | A-2 | + | B-3 | + | C-4 | = 0, find the value of 2A + B + C

1 because - (+ 7) (2) = - 7 * 2 = - 14 [- (- 3)] (3) = 3 * 3 = 9 | + [- (- 200] | (4) = 200 * 4 = 800 [- (- 1 / 70] = 1 / 70, so - (+ 7) (2) - [- (- 3)] (3) - | + [- (- 200] | (4) + [- (- 1 / 70] = - 14-9-800 + 1 / 70 = - 57609 / 702 because the absolute values are all non negative, we should make | A-2 | + | B -

First, simplify, then find the direct (1 + a) (1-A) + a (A-2), where 1 / 2

=1-a^2+a^2-2a
=1-2a
=1-1
=0
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If the math problem is right, add more points, simplify or calculate
1. (radical 7 + radical 3) x (radical 7-radical 3) - radical 16
2. (2-Square of radical 10) + radical 40
3. Radical 2 / 3 - radical 216 + 42 radical 1 / 6

1、（√7+√3)×（√7-√3)-√16 = (7-3)-4 = 0
2、[2-(√10)^2]+√(40)
=(2-10)+2√10=-8+2√10
3、√(2/3)-√(216)+42√(1/6)
=(√6)÷3-6√6+7√6
=(2/3)×√6
Please, choose me. I'm pathetic. His second question is wrong

Simplified
（x-a）*0.8=b,
Calculate the value of X

0.8x-0.8a=b
0.8x=b+0.8a
x=b+0.8a/0.8

A mathematical problem, simplified, as follows
a(a+b+c)-b(a-b-c)-c(a+b+c)
Draw to the simplest
Well, let's ask another math problem, an applied problem of quadratic equation of two variables,
It is known that the sum of the ages of a and B this year is 50. When a is as old as B, the age of a is twice that of B. how old are a and B this year?
Write down the process of solving the problem and the idea of doing it,

1. A (a + B + C) - B (a-b-c) - C (a + B + C) = a ^ 2 + AB + ac-ab + B ^ 2 + bc-ac-bc-c ^ 2 = a ^ 2 + B ^ 2-C ^ 22. Suppose a's age is x and B's age is y, then x + y = 50 The sum of their ages was 50 years old, y = 2 * (Y - (X-Y)) On the left side of the equation is the numerical expression of "a is as old as B", on the right side: X-Y