Solve for x: log₂(x-1) + log₂(x+2) = 2 — JEE Mathematics
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
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Discussion (3)
J
Brilliant explanation, the substitution step is what I kept missing.
KS
How do we know the approximation is valid here?
NA
Thanks a ton, I was stuck on this exact problem for an hour.