here we need to find the value of cot 9° cot 27° cot 63° cot 81°.................(1)
cot 9° = cot (90 - 81)° = tan 81°............(2)
cot 27° = cot (90 - 63)° = tan 63°...............(3)
using equations 1, 2 and 3
cot 9° cot 27° cot 63° cot 81° = tan 81° tan 63° cot 63° cot 81° = 1
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