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What is (tan x + tan y)(1 – cot x cot y) + (cot x + cot y)(1 – tan x tan y) equal to?

This question was previously asked in
CDS I 2020 Elementary Mathematics Previous Year Paper (02-Feb-2020)
The correct answer is

0

Given:

(tan x + tan y)(1 – cot x cot y) + (cot x + cot y)(1 – tan x tan y)

Used formula:

\(\frac{\tan x+\tan y}{1-\tan x\tan y} =\frac{\cot x +\cot y}{\cot x \cot y -1}\)

Calculation:

(tan x + tan y)(1 – cot x cot y) + (cot x + cot y)(1 – tan x tan y)

∵ \(\frac{\tan x+\tan y}{1-\tan x\tan y} =\frac{\cot x +\cot y}{\cot x \cot y -1}\)

∴ tan x + tan y = \(\frac{(\cot x +\cot y)(1 - \tan x \tan y)}{\cot x \cot y -1}\)

\(\frac{(\cot x +\cot y)(1 - \tan x \tan y)}{\cot x \cot y -1}\) × (1 – cot x cot y)  + (cot x + cot y)(1 – tan x tan y)

\(-\frac{(\cot x +\cot y)(1 - \tan x \tan y)}{1-\cot x \cot y }\)×  (1 – cot x cot y)   +  (cot x + cot y)(1 – tan x tan y)

= - (cot x + cot y)(1 – tan x tan y) + (cot x + cot y)(1 – tan x tan y)

= 0

∴ The value of (tan x + tan y)(1 – cot x cot y) + (cot x + cot y)(1 – tan x tan y) is 0.

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