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Q.

The value of tan 20° tan 80° cot 50° is ____.


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Detailed Solution

We know that,
tan (a + b) = tan a + tan b  1-tan atan b   and tan (a - b) = tan a - tan b  1 + tan atan b  
Thus, tan 20° tan 80° cot 50° = tan (30 – 10) ° tan (90 – 10) ° .1tan 50°  = tan (30 – 10) ° tan (90 – 10) ° .1tan 50° 
= tan (30 – 10) °. cot 10 ° .1tan 60 - 10° 
=tan 30° - tan 10°  1 + tan 30° tan 10°  . 1tantan 10° . 1 + tan 60° tan 10°  tan 60°- tan 10°  
=  - tan 10°  1 +  tan 10°  . 1tan 10° . 1 +  tan 10°  - tan 10°  
= -  tan 10°   + tan 10°  . 1tan 10° . 1 +  tan 10°  - tan 10°  
=1 - 310° 3 - tan210°. 1tantan 10° 
=1 - 310° 3tantan 10°  - tan310°  =1 tantan 3(10°)       [tan (3A) = 3tantan A  - 31 - 3 A ]
 =1 tantan 30°  
 =cotcot 30° 
=3  
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