Question
1+cos 56∘+cos 58∘−cos 66∘=
Answer: Option C
:
C
1+cos56∘+cos58∘−cos66∘=
=2cos228∘+2sin62∘.sin4∘
=2cos228∘+2cos28∘.sin4∘
=2cos28∘(cos28∘+cos86∘)
=2cos28∘.2cos57∘.cos29∘
=4cos28∘cos29∘sin33∘
Aliter: Apply the condition identify
cosA+cosB−cosC=−1+4cosA2cosB2sinc2
[∴56∘+58∘+66∘=180∘]
We get the value of required expression equal to 4cos28∘cos29∘sin33∘.
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:
C
1+cos56∘+cos58∘−cos66∘=
=2cos228∘+2sin62∘.sin4∘
=2cos228∘+2cos28∘.sin4∘
=2cos28∘(cos28∘+cos86∘)
=2cos28∘.2cos57∘.cos29∘
=4cos28∘cos29∘sin33∘
Aliter: Apply the condition identify
cosA+cosB−cosC=−1+4cosA2cosB2sinc2
[∴56∘+58∘+66∘=180∘]
We get the value of required expression equal to 4cos28∘cos29∘sin33∘.
Was this answer helpful ?
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