Prove \(\displaystyle{{\tan{{70}}}^{\circ}-}{{\tan{{50}}}^{\circ}+}{{\tan{{10}}}^{\circ}=}\sqrt{{{3}}}\)

Milton Robertson

Milton Robertson

Answered question

2022-03-30

Prove
tan70tan50+tan10=3

Answer & Explanation

Raiden Griffin

Raiden Griffin

Beginner2022-03-31Added 13 answers

Consider tanx+tan(x+60)+tan(x+120), then
tanx+tan(x+60)+tan(x+120)=tanx+tanx+tan601tanxtan60+tanx+tan1201tanxtan120
=tanx+tanx+313tanx+tanx31+3tanx
=tanx+tanx+3tan2x+3+3tanx13tan2x+tan3tan2x3+3tanx13tan2x
=tanx(13tan2x)13tan2x+8tanx13tan2x
=9tanx3tan33x13tan2x
=3(3tanxtan33x13tan2x)
=3tan3x
See triple-angle formula of trigonometric identities. Plugging in x=10, we get
tan10+tan(10+60)+tan(10+120)=3tan(310)
tan10+tan70+tan130=3tan(30)
tan10+tan70+tan(18050)=313
tan10+tan70tan50=3

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