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Prove that tan20°tan40°tan80°=tan60°

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Prove that tan20°tan40°tan80°=tan60°
  • 1 answers

Gaurav Seth 6 years, 10 months ago

<pre> tan 40 * tan 80* tan 20=(sin 40 *sin 80) * sin 20 /(cos 40 *cos 80 ) cos 20</pre>

 now, multiplying numerator and denominator by 2,

                                 =(2*sin 40*sin 80) *sin 20 /(2 cos 40*cos 80) cos 20]

                                 =(cos 40- cos 120)*sin 20/( (cos 120+ cos 40) cos 20

                                 =(2 cos 40 +1) sin 20 /(2 cos 40-1) cos 20

                                 =(2 cos 40*sin 20 +sin 20) /(2 cos 40 cos 20 -cos 20)

                                 =(sin 60 -sin 20 +sin 20)/ (cos 60+sin 20-sin 20)

                                = sin 60/cos 60

                                 =tan 60.

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