Question

cos80°+cos40°=cos20°​

Answer 1

Answer:

Explanation:

To prove

cos

80

+

cos

40

−

cos

20

=

0

Apply the following

cos

A

+

cos

B

=

2

cos

(

A

+

B

2

)

cos

(

A

−

B

2

)

cos

60

=

1

2

Therefore,

cos

80

+

cos

40

=

2

cos

(

80

+

40

2

)

cos

(

80

−

40

2

)

=

2

cos

60

cos

20

=

2

⋅

1

2

cos

20

=

cos

20

So,

cos

80

+

cos

40

−

cos

20

=

cos

20

−

cos

20

=

0

Answer 2

Answer:

Step-by-step explanation:

by applying formulae COS C +COS D = 2COS (C+D/2) COS (C-D/2)

COS 80 + COS 40 WILL EQUAL TO =

2 COS (120/2) COS (80-40/2)=COS 40

HENCE PROVED