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Exercise 1670Algebra › Equations and radicalsOlympiadAdded 3 Nov 2026

An equation with the floor function

#floor function#integer part#olympiad

Statement

Solve in R\mathbb{R} the equation
⌊x⌋+x=5.\lfloor x\rfloor + x = 5.

Hint

Write x=n+rx=n+r with n=⌊x⌋∈Zn=\lfloor x\rfloor\in\mathbb{Z} and r∈[0,1[r\in[0,1[.

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Solution

Notation. x=n+rx=n+r with n=⌊x⌋n=\lfloor x\rfloor, 0≤r<10\le r<1. The equation is
n+(n+r)=5  ⟺  2n+r=5.n+(n+r)=5\iff2n+r=5.

Consequences. Since 0≤r<10\le r<1, we have 5−1<2n≤55-1<2n\le5, i.e. 2<n≤2.52< n\le2.5. Since n∈Zn\in\mathbb{Z}, n=2n=2.

Then. r=5−2n=5−4=1r=5-2n=5-4=1. But r<1r<1, contradiction. So no solution.

Conclusion. No real solution.