⌊ 3 2 x + 1 ⌋ + ⌊ 6 4 x + 5 ⌋ = 2 3 x − 1 Find the number of real values of x satisfying the above equation.
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Is there a formal proof for that L H S < R H S for n > 1 0 ?
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No, but it is obvious that LHS decreases faster than RHS.
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We know that the LHS increase with n although it is smaller than its continuous form as follows.
⌊ 9 4 n + 5 ⌋ + ⌊ 1 8 8 n + 1 9 ⌋ ≤ 9 4 n + 5 + 1 8 8 n + 1 9
Consider the following:
9 4 n + 5 + 1 8 8 n + 1 9 9 8 n + 1 8 2 9 ≤ n 9 1 n ≥ 1 8 2 9 n ≥ 1 4 . 5 ≤ n
Therefore, the LHS is smaller than RHS at an n . In this case it is n = 1 1 .
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Since the LHS is an integer, the RHS must also be an integer n , ⇒ 2 3 x − 1 = n and x = 3 2 n + 1 . And that:
⌊ 3 2 x + 1 ⌋ + ⌊ 6 4 x + 5 ⌋ ⌊ 3 2 ( 3 2 n + 1 ) + 1 ⌋ + ⌊ 6 4 ( 3 2 n + 1 ) + 5 ⌋ ⌊ 9 4 n + 5 ⌋ + ⌊ 1 8 8 n + 1 9 ⌋ ⌊ 9 4 n + 5 ⌋ + ⌊ 1 8 8 n + 1 ⌋ + 1 = 2 3 x − 1 = n = n = n
We note that the LHS increases with n and LHS > RHS for n ≤ 1 and LHS < RHS for n ≥ 1 1 . And when:
⎩ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎧ n = 2 , n = 3 , n = 4 , n = 5 , n = 6 , n = 7 , n = 8 , n = 9 , n = 1 0 , ⇒ 1 + 0 + 1 = 2 , ⇒ 1 + 1 + 1 = 3 , ⇒ 2 + 1 + 1 = 4 , ⇒ 2 + 2 + 1 = 5 , ⇒ 3 + 2 + 1 = 6 , ⇒ 3 + 3 + 1 = 7 , ⇒ 4 + 3 + 1 = 8 , ⇒ 4 + 4 + 1 = 9 , ⇒ 5 + 4 + 1 = 1 0 , acceptable acceptable acceptable acceptable acceptable acceptable acceptable acceptable acceptable
There are 9 real values of x satisfying the equation.