Question

Cos6cos42cos66cos78=1/6

Answer 1

prove (cos 6) (cos 42)cos 66 (cos 78)=1/16 
Sol LS= {cos 6* cos 66}{cos 42* cos 78} 
= 1/4{2cos 6* cos 66}{2cos 42* cos 78} 
= 1/4{ cos 72 + cos 60}{cos 120 + cos 36} 
= 1/4{ sin 18 +1/2}{-1/2 + cos 36} 
= 1/4{ (sqrt 5 -1)/4 +1/2} {-1/2+ (sqrt 5 +1)/4} , 
because sin 18 = {(sqrt 5)- 1}/4 and cos 36 = {(sqrt 5)+ 1}/4 
= 1/4[{ (sqrt 5)+1}/4] [{ (sqrt 5 -1)/4}] 
= 1/4[5-1]/16 = 1/4(4/16) = 1/16 = RS 

Answer 2

Step-by-step explanation:

Hope it helps.

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