Actually, the correct identity is: cot(x + y) = (cot x cot y - 1) / (cot x + cot y) This identity can be derived from the sum formulas for tangent and cotangent: tan(x + y) = (tan x + tan y) / (1 - tan x tan y) cot(x + y) = (cot x cot y - 1) / (cot x + cot y) We can start by substituting cot(x + y) with its equivalent expression: cot(x + y) = cot x cot y - 1 / (cot x + cot y) Multiplying both sides by (cot x + cot y), we get: cot(x + y) (cot x + cot y) = cot x cot y - 1 Expanding the left side, we get: cot x cot y + cot x cot