We are tasked with finding the minimum possible value of nnn such that the equation
⌊n4⌋−⌊n9⌋=2\left\lfloor \frac{n}{4} \right\rfloor - \left\lfloor \frac{n}{9} \right\rfloor = 2⌊4n⌋−⌊9n⌋=2
holds, where ⌊x⌋\left\lfloor x \right\rfloor⌊x⌋ denotes the greatest integer less than or equal to xxx.
Step 1: Understanding the equation
Let ⌊n4⌋=a\left\lfloor \frac{n}{4} \right\rfloor = a⌊4n⌋=a and ⌊n9⌋=b\left\lfloor \frac{n}{9} \right\rfloor = b⌊9n⌋=b. The equation becomes:
a−b=2.a - b = 2.a−b=2.
This implies:
a=b+2.a = b + 2.a=b+2.
Thus, we need to find nnn such that the greatest integer less than or equal to n4\frac{n}{4}4n is 2 greater than the greatest integer less than or equal to n9\frac{n}{9}9n.
Step 2: Analyze intervals for nnn
- a=⌊n4⌋a = \left\lfloor \frac{n}{4} \right\rfloora=⌊4n⌋ means that 4a≤n<4(a+1)4a \leq n < 4(a+1)4a≤n<4(a+1).
- b=⌊n9⌋b = \left\lfloor \frac{n}{9} \right\rfloorb=⌊9n⌋ means that 9b≤n<9(b+1)9b \leq n < 9(b+1)9b≤n<9(b+1).
We are looking for values of nnn such that:
a=b+2.a = b + 2.a=b+2.
Thus, we have:
⌊n4⌋=⌊n9⌋+2.\left\lfloor \frac{n}{4} \right\rfloor = \left\lfloor \frac{n}{9} \right\rfloor + 2.⌊4n⌋=⌊9n⌋+2.
Step 3: Try different values of nnn
Let’s try different values of nnn and calculate ⌊n4⌋\left\lfloor \frac{n}{4} \right\rfloor⌊4n⌋ and ⌊n9⌋\left\lfloor \frac{n}{9} \right\rfloor⌊9n⌋ to find the smallest nnn that satisfies the equation.
For n=7n = 7n=7:
- ⌊74⌋=1\left\lfloor \frac{7}{4} \right\rfloor = 1⌊47⌋=1,
- ⌊79⌋=0\left\lfloor \frac{7}{9} \right\rfloor = 0⌊97⌋=0,
- 1−0=1≠21 - 0 = 1 \neq 21−0=1=2, so n=7n = 7n=7 does not work.
For n=8n = 8n=8:
- ⌊84⌋=2\left\lfloor \frac{8}{4} \right\rfloor = 2⌊48⌋=2,
- ⌊89⌋=0\left\lfloor \frac{8}{9} \right\rfloor = 0⌊98⌋=0,
- 2−0=22 - 0 = 22−0=2, which satisfies the equation.
Thus, the minimum possible value of nnn is 8\boxed{8}8.
The correct answer is B. 8.
Bunuel
Let [x] represent the greatest integer less than or equal to x. If n is a positive integer such that [n/4] - [n/9] = 2, what is the minimum possible value of n?
A. 7
B. 8
C. 9
D. 12
E. 19