C2 Trig Question....

__WiZaRD__

Given that angle A is obuse and cosA = -√(7/11), show that tanA = (-2√7)/7

Thanx

Reply 1

Malik

__WiZaRD__

Given that angle A is obuse and cosA = -√(7/11), show that tanA = (-2√7)/7

Thanx

lol hard at work on xmas day :p:

sin²A=1-cos²A
sin²A=1-(-√7/11)²
sin²A=1-(7/11) = 4/11
sin A= ±2/√11 because A is obtuse it will be positive so
sin A=2/√11

tan A= sin A/cos A
=(2/√11)/(-7/√11)
= -2/√7
= (-2√7)/7

Reply 2

Christophicus

__WiZaRD__

Given that angle A is obuse and cosA = -√(7/11), show that tanA = (-2√7)/7

Thanx

1+tan²A = sec²A = 1/cos²A
tan²A = 1/cos²A - 1
tan²A = 1/7/11 - 1
tan²A = 11/7 - 7/7 = 4/7
tanA = ±√(4/7)

A is obtuse => A lies in Q2 (90<A<180)
cosA and tanA are -ve in Q2

Therefore take the negative route.
tanA = -√(4/7) = -2/√7 = -2√7/7

Reply 3

mizfissy815

Given that cosA = -√(7/11)
Then Adjacent= -√7
Hypotenuse= √11
Using Pythagoras theorem -> a² +b² = c²

Opposite= √ [(√11)² - (-√7)²]= 2

So tanA= Opposite/Adjacent
TanA= 2/(-√7)....rationalise by multiply top and bottom by √7.
TanA= -(2√7)/7

Reply 4

Malik

mizfissy815

Given that cosA = -√(7/11)
Then Adjacent= -√7
Hypotenuse= √11
Using Pythagoras theorem -> a² +b² = c²

Opposite= (√11)² - (-√7)² = 2

So tanA= Opposite/Adjacent
TanA= 2/(-√7)....rationalise by multiply top and bottom by √7.
TanA= -(2√7)/7

Opposite= (√11)² - (-√7)² ≠ 2

Reply 5

mizfissy815

oops my bad....edited...I forgot the √.

Reply 6

__WiZaRD__

Hey guys i have another problem :mad:

... Here it is:

a) Determine the solutions of the equation cos(2x-30)=0 for which 0 « x « 360

Reply 7

Esquire

that is such a standard question... read the book before you post here lol.

Reply 8

__WiZaRD__

Esquire

that is such a standard question... read the book before you post here lol.

thats a bit rude...

Im not as lazy as you make out, i have read the book and gone over the whole chapter quite well, and its not that i dont know how to do any of it its just that i keep getting the answer for this one wrong, i can do the rest of these types fine.

Anyone?

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oh and by the way, maths is not one of those subjects which you can just read the textbook for and be expected to solve any question! (or ast least not for me)

Reply 9

mizfissy815

cos(2x-30)=0
cos Φ = 0 ....... -30 « Ø « 690
Φ= 90, 270, 450, 630
2x-30= 90, 270, 450, 630
x= 60, 150, 240, 330

ahh, thx

omg sorry... i said "lol"... what I said was meant to have been taken with a pinch of salt

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but seriously didnt you get the heinemann C2 book for your course?

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and i know that maths is not a subject that you can read the book and expect to solve every question from that, but, as i said, it is a standard question.