Discussion

Explanation:

log₁₃ log₂₁ {$\sqrt{x+21}$ + $\sqrt{x}$} = 0

∴ log₂₁ {$\sqrt{x+21}$ + $\sqrt{x}$} = 13° = 1

∴ {$\sqrt{x+21}$ + $\sqrt{x}$} = 21¹ = 21

∴ $\sqrt{x+21}$= 21 - $\sqrt{x}$

Squaring both sides,

x + 21 = 441 + x - 42$\sqrt{x}$

∴ 42$\sqrt{x}$ = 420

∴ x = 100

Hence, option (d).

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