JEE mathematics

If $\log_a(bc) = x$, $\log_b(ca) = y$, and $\log_c(ab) = z$, find the value of $\frac{1}{x+1} + \frac{1}{y+1} + \frac{1}{z+1}$.

EEthanWalker47 Asked 2mo ago 1,493 views 1 answer

If $\log_a(bc) = x$, $\log_b(ca) = y$, and $\log_c(ab) = z$, find the value of $\frac{1}{x+1} + \frac{1}{y+1} + \frac{1}{z+1}$.

1 Answer

VAVidaara Admin ✓ Vidaara Team ✓ Accepted · 2mo ago ▲ 9

Add 1 to $x$:

$$x + 1 = \log_a(bc) + \log_a a = \log_a(abc) \implies \frac{1}{x+1} = \log_{abc} a$$

Similarly:

$$y + 1 = \log_b(ca) + \log_b b = \log_b(abc) \implies \frac{1}{y+1} = \log_{abc} b$$

$$z + 1 = \log_c(ab) + \log_c c = \log_c(abc) \implies \frac{1}{z+1} = \log_{abc} c$$

Summing the reciprocals:

$$\frac{1}{x+1} + \frac{1}{y+1} + \frac{1}{z+1} = \log_{abc} a + \log_{abc} b + \log_{abc} c = \log_{abc}(abc) = 1$$

Answer: 1

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Discussion (4)

J
Is there a faster shortcut for this in the actual exam? Time is tight.
JulienMoreau51 · 2mo ago
SD
Is there a faster shortcut for this in the actual exam? Time is tight.
Siddharth Das · 2mo ago
SG
Plotting it roughly also helps you sanity-check the sign.
Sneha Gupta · 2mo ago
VA
Great discussion here. If you want more practice on this concept, check the related questions in this category.
Vidaara Admin · Vidaara Team · 2mo ago
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