Without trigonometric tables, prove that:\left( \sin { { 72 }^{ o } } +\cos { { 18 }^{ o } }  \right) \left( \sin { { 72 }^{ o } } -\cos { { 18 }^{ o } }  \right) =0

Foundation

Mathematics Foundation

Introduction to Trigonometry

Compound Angle

Properties of Triangle

Question

Without trigonometric tables, prove that:

\left( \sin { { 72 }^{ o } } +\cos { { 18 }^{ o } }&nbsp; \right) \left( \sin { { 72 }^{ o } } -\cos { { 18 }^{ o } }&nbsp; \right) =0

College PredictorLive

Know your College Admission Chances Based on your Rank/Percentile, Category and Home State.

Get your JEE Main Personalised Report with Top Predicted Colleges in JoSA

L H S = (\sin 72^{o} + \cos 18^{o},) (\sin 72^{o} - \cos 18^{o},) = \sin^{2} 72^{o} - \cos^{2} (90^{o} - 55^{o}),

= \sin^{2} 72^{o} - \sin^{2} 72^{o} = 0 = R H S