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← JEE Advanced 2021 · Paper 1
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JEE Advanced 2021 · Paper 1, Mathematics question 18

In a triangle ABCABC, let AB=23AB = \sqrt{23}, BC=3BC = 3 and CA=4CA = 4. Then the value of
cot⁡A+cot⁡Ccot⁡B\frac{\cot A + \cot C}{\cot B}
is ____.

Answer and worked solution

Official answer: 2

Cotangents in terms of the sides. With area Δ\Delta, cot⁡A=b2+c2−a24Δ\cot A=\dfrac{b^2+c^2-a^2}{4\Delta}, and similarly for BB and CC. Therefore
cot⁡A+cot⁡Ccot⁡B=2b2a2+c2−b2.\frac{\cot A+\cot C}{\cot B}=\frac{2b^2}{a^2+c^2-b^2}.

Substitute a=BC=3a=BC=3, b=CA=4b=CA=4, c=AB=23c=AB=\sqrt{23}:
329+23−16=2.\frac{32}{9+23-16}=2.