Question

cos20 cos40 cos80=1/16

Answer 1

see my answer !!!!!!!!

I think question should be

cos20 cos40 cos 60 cos80 = 1/16

you didn't write cos60

no problems

don't do next time wrong

OK

Answer 2

Hey there !!!!!

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=cos20cos40cos60cos80 

Multiply and divide with 2sin20 

=2sin20cos20cos40cos60cos80/2sin20

We know that 2sinAcosA = sin 2A

So 2sin20cos20 = sin2*20 =sin40

= sin40cos40cos60cos80/2sin20

Multiply and divide with "2"

= 2sin40cos40cos60cos80/2*2sin20

=cos60sin80cos80/2*2sin20

Multiply and divide with "2"

= cos60*2sin80cos80/8sin20

= cos60sin160/8sin20

We know that 

sin(180-∅) = sin∅

So sin160 = sin(180-20) = sin20

So,

= cos60sin160/8sin20

=cos60sin20/8sin20

= cos60/8

cos60 = 1/2

= 1/16

LHS = RHS