Maths WorkshopMaths problems with full solutions
Exercise 1225Algebra › Equations and radicalsOlympiadAdded 4 Nov 2026

The integer part of a telescoping sum

#recurrence#telescoping#floor#monotone sequence

Statement

Define x1=12x_1=\dfrac12 and xn+1=xn2+xnx_{n+1}=x_n^2+x_n. Compute
⌊∑k=1202511+xk⌋.\left\lfloor\sum_{k=1}^{2025}\frac1{1+x_k}\right\rfloor.

Hint

From xk+1=xk(xk+1)x_{k+1}=x_k(x_k+1), express 11+xk\frac1{1+x_k} as a difference of two consecutive terms 1xk−1xk+1\frac1{x_k}-\frac1{x_{k+1}}.

Show solutionHide solution

Solution

Telescoping. Since xk+1=xk(1+xk)x_{k+1}=x_k(1+x_k),
1xk−1xk+1=xk+1−xkxkxk+1=11+xk.\frac1{x_k}-\frac1{x_{k+1}}=\frac{x_{k+1}-x_k}{x_kx_{k+1}}=\frac1{1+x_k}.
Hence
∑k=1202511+xk=1x1−1x2026=2−1x2026.\sum_{k=1}^{2025}\frac1{1+x_k}=\frac1{x_1}-\frac1{x_{2026}}=2-\frac1{x_{2026}}.

Size of x2026x_{2026}. The sequence is positive and increasing (xn+1−xn=xn2>0x_{n+1}-x_n=x_n^2>0). Moreover x2=34x_2=\frac34 and x3=34⋅74=2116>1x_3=\frac34\cdot\frac74=\frac{21}{16}>1. So x2026>x3>1x_{2026}>x_3>1, hence 0<1x2026<10<\frac1{x_{2026}}<1.

Conclusion. The sum lies in (1,2)(1,2), so its integer part is 1\boxed1.

Check. With only 66 terms the sum is 2−1x7≈1.999962-\frac1{x_7}\approx1.99996.

Lesson. When a recurrence has the form xn+1=xn(xn+c)x_{n+1}=x_n(x_n+c), the quantity 1xn+c\frac1{x_n+c} telescopes.