JEE mathematics

Solve for x: log₂(x-1) + log₂(x+2) = 2 — JEE Mathematics

VVivaanJoshi73 Asked 14d ago 1,417 views 1 answer

Solve for $x$: $\log_2(x-1) + \log_2(x+2) = 2$.

1 Answer

SSandeepRanasinghe88 ✓ Accepted · 12d ago ▲ 25

Domain constraints:

  • $x - 1 > 0 \implies x > 1$
  • $x + 2 > 0 \implies x > -2$
    Domain: $x > 1$.

Use the product rule for logarithms:

$$\log_2[(x-1)(x+2)] = 2$$

Convert from logarithmic to exponential form:

$$(x-1)(x+2) = 2^2$$

$$x^2 + x - 2 = 4$$

$$x^2 + x - 6 = 0$$

$$(x+3)(x-2) = 0 \implies x = -3 or x = 2$$

Since we require $x > 1$, we discard $x = -3$.
Thus, $x = 2$.

Answer: 2

Log in to post your own answer or join the discussion.

Discussion (3)

J
Brilliant explanation, the substitution step is what I kept missing.
JulienMoreau51 · 11d ago
KS
How do we know the approximation is valid here?
Karan Singh · 10d ago
NA
Thanks a ton, I was stuck on this exact problem for an hour.
Nisha Agarwal · 8d ago
← Back to all questions