A geometry problem by prasanth pp

Geometry Level 3

cos 2 ( 7 6 ) + cos 2 ( 1 6 ) cos ( 7 6 ) × cos ( 1 6 ) = ? \large \cos^2(76^\circ) + \cos^2(16^\circ) -\cos(76^\circ) \times \cos(16^\circ) = \, ?


The answer is 0.75.

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2 solutions

Prasanth Pp
Mar 24, 2016

Given c o s 2 ( 76 ) + c o s 2 ( 16 ) c o s ( 76 ) × c o s ( 16 ) cos^2(76) + cos^2(16) - cos(76) \times cos(16) = 1 2 × ( 2 c o s 2 ( 76 ) + 2 c o s 2 ( 16 ) 2 c o s ( 76 ) × c o s ( 16 ) ) \frac{1}{2} \times (2cos^2(76) + 2cos^2(16) - 2cos(76) \times cos(16)) = 1 2 × ( 1 + c o s ( 152 ) + 1 + c o s ( 32 ) c o s ( 92 ) c o s ( 60 ) ) \frac{1}{2} \times (1+cos(152) + 1+cos(32) - cos(92) - cos(60)) = 1 2 × ( 2 c o s ( 60 ) + c o s ( 152 ) + c o s ( 32 ) c o s ( 92 ) ) \frac{1}{2} \times (2-cos(60)+cos(152) +cos(32) - cos(92) ) = 1 2 × ( 2 c o s ( 60 ) + 2 c o s ( 152 + 32 2 ) × c o s ( 152 32 2 ) c o s ( 92 ) ) \frac{1}{2} \times (2-cos(60)+2cos(\frac{152+32}{2}) \times cos(\frac{152-32}{2}) - cos(92) ) = 1 2 × ( 2 c o s ( 60 ) + 2 c o s ( 184 2 ) × c o s ( 120 2 ) c o s ( 92 ) ) \frac{1}{2} \times (2-cos(60)+2cos(\frac{184}{2}) \times cos(\frac{120}{2}) - cos(92) ) = 1 2 × ( 2 c o s ( 60 ) + 2 c o s ( 92 ) × c o s ( 60 ) c o s ( 92 ) ) \frac{1}{2} \times (2-cos(60)+2cos(92) \times cos(60) - cos(92) ) = 1 2 × ( 2 1 2 + 2 c o s ( 92 ) × 1 2 c o s ( 92 ) ) \frac{1}{2} \times (2-\frac{1}{2}+2cos(92) \times \frac{1}{2} - cos(92) ) = 1 2 × ( 2 1 2 + c o s ( 92 ) c o s ( 92 ) ) \frac{1}{2} \times (2-\frac{1}{2}+cos(92) - cos(92) ) = 1 2 × ( 2 1 2 ) \frac{1}{2} \times (2-\frac{1}{2} ) = 1 2 × ( 3 2 ) \frac{1}{2} \times (\frac{3}{2} ) = 3 4 \frac{3}{4} =0.75

Ahmad Saad
Mar 25, 2016

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