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secondary 3 | A Maths
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trish
Trish

secondary 3 chevron_right A Maths chevron_right Singapore

sorry i need help with this, thank you!

Date Posted: 4 years ago
Views: 404
J
J
4 years ago
cos 3A

= cos (2A + A)

= cos2A cosA - sin2A sinA

= (2cos²A - 1)(cosA) - 2sinAcosA(sinA)

= 2cos³A - cosA - 2sin²A cosA

= 2cos³A - cosA - 2(1 - cos²A)(cosA)

= 2 cos³A - cosA - 2cosA + 2cos³A

= 4 cos³A - 3 cosA (shown)





cos⁴A - sin⁴A

= (cos²A)² - (sin²A)²

= (cos²A + sin²A)(cos²A - sin²A)

= (1) cos2A

= cos2A (shown)
Eric Nicholas K
Eric Nicholas K
4 years ago
Sending you in half an bour
Eric Nicholas K
Eric Nicholas K
4 years ago
Second one just factorise by a2 - b2 = (a + b) (a - b)
J
J
4 years ago
Important thing : be familiar with your trigo identities.

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Eric Nicholas K
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Something like this would be good.