What is an approximate sum of the sequence

Algebra Level 3

1 2 + 2 3 + 3 4 + 4 5 + + 99 100 = ? \frac 12 + \frac 23 + \frac 34 + \frac 45 + \cdots + \frac {99}{100} = ?

Give your answer to 2 decimal places.


The answer is 94.81.

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4 solutions

Chew-Seong Cheong
Jul 18, 2017

Relevant wiki: Harmonic Number

S = 1 2 + 2 3 + 3 4 + 4 5 + + 99 100 = 1 1 2 + 1 1 3 + 1 1 4 + + 1 1 100 = 99 ( 1 2 + 1 3 + 1 4 + + 1 100 ) = 100 ( 1 + 1 2 + 1 3 + 1 4 + + 1 100 ) = 100 H 100 H n is the n th harmonic number. 100 ln 100 γ γ 0.5772 is the Euler-Mascheroni constant. 94.82 \begin{aligned} S & = \frac 12 + \frac 23 + \frac 34 + \frac 45 + \cdots + \frac {99}{100} \\ & = 1 - \frac 12 + 1- \frac 13 + 1- \frac 14 + \cdots + 1 - \frac 1{100} \\ & = 99 - \left(\frac 12 + \frac 13 + \frac 14 + \cdots + \frac 1{100} \right) \\ & = 100 - \left(1 + \frac 12 + \frac 13 + \frac 14 + \cdots + \frac 1{100} \right) \\ & = 100 - {\color{#3D99F6}H_{100}} & \small \color{#3D99F6} H_n \text{ is the }n \text{th harmonic number.} \\ & \approx 100 - \ln 100 - {\color{#3D99F6}\gamma} & \small \color{#3D99F6} \gamma \approx 0.5772 \text{ is the Euler-Mascheroni constant.} \\ & \approx \boxed{94.82} \end{aligned}

Md Zuhair
Jul 18, 2017

I couldnt do it with Harmonic numbers as i didnt knew it so i wrote a program

int n,i;
float sum=0; 

cout<<"Input n=";
cin>>n;


for (i=1;i<=n;i++)
    sum += (i*1.0)/(i+1);


cout<<"SUM="<<sum;
fflush(stdin);
getchar();
return 0;

OUTPUT=

1
2
3
4
ans = 0
for denom in range(1, 100):
    ans += denom / (denom + 1)
print(ans)

or just:

1
print(sum(denom / (denom + 1) for denom in range(1, 100))) 

Vijay Simha
Jul 18, 2017

The solution can be found by re-writing the sum as 100 - H(100) where H(100) is the 100th Harmonic number

which approximates to 100 - (Gamma + 1/2*100 + ln(100))

H(n) = Gamma + 1/2*n + ln(n) Where Gamma is the Euler-Mascheroni constant .

Well, this is not correct or exact. First, you should specify that it should only be correct to 3 decimal places.

Also, your answer is wrong. Rounded to the nearest thousandth, the answer is 94.813.

Siva Budaraju - 3 years, 10 months ago

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