Olympiad Maths Prep

Track / Stage 6 / 326 of 400 #1326 of 2000

Problem 1326

National olympiad, first round
Algebra Difficulty 6.6 Find the answer

Which of the following rationals is greater ?

19951994+119951995+1\frac{1995^{1994} + 1}{1995^{1995} + 1} or 19951995+119951996+1\frac{1995^{1995} + 1}{ 1995^{1996} +1}

Official solution

1. Let's denote x=1995 x = 1995 . We need to compare the two fractions:
x1994+1x1995+1andx1995+1x1996+1 \frac{x^{1994} + 1}{x^{1995} + 1} \quad \text{and} \quad \frac{x^{1995} + 1}{x^{1996} + 1}

2. Consider the function f(x)=xn+1xn+1+1 f(x) = \frac{x^n + 1}{x^{n+1} + 1} . We need to show that this function is decreasing for x>0 x > 0 .

3. To show that f(x) f(x) is decreasing, we can compute its derivative and show that it is negative for x>0 x > 0 .

4. The derivative of f(x) f(x) with respect to x x is:
f(x)=ddx(xn+1xn+1+1) f'(x) = \frac{d}{dx} \left( \frac{x^n + 1}{x^{n+1} + 1} \right)

5. Using the quotient rule, we have:
f(x)=(xn+1+1)ddx(xn+1)(xn+1)ddx(xn+1+1)(xn+1+1)2 f'(x) = \frac{(x^{n+1} + 1) \cdot \frac{d}{dx}(x^n + 1) - (x^n + 1) \cdot \frac{d}{dx}(x^{n+1} + 1)}{(x^{n+1} + 1)^2}

6. Simplifying the derivatives:
ddx(xn+1)=nxn1 \frac{d}{dx}(x^n + 1) = n x^{n-1}
ddx(xn+1+1)=(n+1)xn \frac{d}{dx}(x^{n+1} + 1) = (n+1) x^n

7. Substituting these into the derivative expression:
f(x)=(xn+1+1)nxn1(xn+1)(n+1)xn(xn+1+1)2 f'(x) = \frac{(x^{n+1} + 1) \cdot n x^{n-1} - (x^n + 1) \cdot (n+1) x^n}{(x^{n+1} + 1)^2}

8. Simplifying the numerator:
f(x)=nxn1xn+1+nxn1(n+1)xnxn(n+1)xn(xn+1+1)2 f'(x) = \frac{n x^{n-1} x^{n+1} + n x^{n-1} - (n+1) x^n x^n - (n+1) x^n}{(x^{n+1} + 1)^2}
f(x)=nx2n+nxn1(n+1)x2n(n+1)xn(xn+1+1)2 f'(x) = \frac{n x^{2n} + n x^{n-1} - (n+1) x^{2n} - (n+1) x^n}{(x^{n+1} + 1)^2}
f(x)=nx2n+nxn1nx2nx2n(n+1)xn(xn+1+1)2 f'(x) = \frac{n x^{2n} + n x^{n-1} - n x^{2n} - x^{2n} - (n+1) x^n}{(x^{n+1} + 1)^2}
f(x)=x2n+nxn1(n+1)xn(xn+1+1)2 f'(x) = \frac{- x^{2n} + n x^{n-1} - (n+1) x^n}{(x^{n+1} + 1)^2}

9. Since x>0 x > 0 , the term x2n - x^{2n} dominates, making f(x)<0 f'(x) < 0 . Therefore, f(x) f(x) is a decreasing function for x>0 x > 0 .

10. Applying this result to our specific case with x=1995 x = 1995 , n=1994 n = 1994 , and n+1=1995 n+1 = 1995 :
19951994+119951995+1>19951995+119951996+1 \frac{1995^{1994} + 1}{1995^{1995} + 1} > \frac{1995^{1995} + 1}{1995^{1996} + 1}

Conclusion:
19951994+119951995+1>19951995+119951996+1 \boxed{\frac{1995^{1994} + 1}{1995^{1995} + 1} > \frac{1995^{1995} + 1}{1995^{1996} + 1}}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.