Source: Extra Practice

Solve the logarithmic equation for xx: log5(x24x5)=log5(2x10)+1\log_5 (x^2 - 4x - 5) = \log_5 (2x - 10) + 1.

Options

Option A is correct

x=9x = 9 only

Option B

x=5x = 5 and x=9x = 9

Option C

x=5x = 5 only

Option D

No real solution

Explanation

First, we can write the right side of the equation as a single logarithm using the property logba+logbc=logb(ac)\log_b a + \log_b c = \log_b(ac): log5(2x10)+1=log5(2x10)+log55=log5[5(2x10)]=log5(10x50)\log_5 (2x - 10) + 1 = \log_5 (2x - 10) + \log_5 5 = \log_5 [5(2x - 10)] = \log_5 (10x - 50).

Now, equate the arguments of the logarithms on both sides: x24x5=10x50x^2 - 4x - 5 = 10x - 50 x214x+45=0x^2 - 14x + 45 = 0 (x5)(x9)=0    x=5 or x=9(x-5)(x-9) = 0 \implies x = 5 \text{ or } x = 9.

Next, we must check these potential solutions against the domain of the original logarithmic terms:

  • For x=5x = 5, the term log5(2x10)\log_5(2x-10) becomes log5(0)\log_5(0), which is undefined because the argument of a logarithm must be strictly positive. Thus, x=5x = 5 is an extraneous root.
  • For x=9x = 9, both arguments x24x5=81365=40>0x^2-4x-5 = 81-36-5 = 40 > 0 and 2x10=1810=8>02x-10 = 18-10 = 8 > 0 are positive. Thus, x=9x = 9 is the only valid solution.