C1/C2 Trig Question HELP!

Given that angle A is reflex and cosA = root13 / 7
(a) find the exact value of sinA
(b) find the exact value of tanA

I generally understand the question but I need someone to explain the angles and signs to me. If angle A is in quadrant 3 then shouldn't cosA= - root13/7?

Reply 1

niamhdoesmaths

21

Original post by Anon736

Given that angle A is reflex and cosA = root13 / 7
(a) find the exact value of sinA
(b) find the exact value of tanA

I generally understand the question but I need someone to explain the angles and signs to me. If angle A is in quadrant 3 then shouldn't cosA= - root13/7?

It'll be in the 4th quadrant surely?

Reply 2

h3rmit

15

Original post by Anon736

Given that angle A is reflex and cosA = root13 / 7
(a) find the exact value of sinA
(b) find the exact value of tanA

I generally understand the question but I need someone to explain the angles and signs to me. If angle A is in quadrant 3 then shouldn't cosA= - root13/7?

Reflex angles are greater than 180 degrees, but are also less than 360 degrees so A could be in the fourth quadrant, making it positive.

Reply 3

Anon736

OP

8

Original post by NiamhM1801

It'll be in the 4th quadrant surely?

Well actually thats a good question because a reflex is greater than 180 so I actually don't know if its quadrant 3 or 4.

Reply 4

B_9710

18

Original post by Anon736

Given that angle A is reflex and cosA = root13 / 7
(a) find the exact value of sinA
(b) find the exact value of tanA

I generally understand the question but I need someone to explain the angles and signs to me. If angle A is in quadrant 3 then shouldn't cosA= - root13/7?

Reflex angle covers both 3rd and 4th quadrant since refle angle is angle bigger than π. So this tells you that in fact angle A is in fact in the 4th quadrant. So sinA will be negative and so will tanA - just sketch the graphs and you will see this.

Reply 5

Anon736

OP

8

Original post by h3rmit

Reflex angles are greater than 180 degrees, but are also less than 360 degrees so A could be in the fourth quadrant, making it positive.

For a question like this how do you know which one to use?

Reply 6

niamhdoesmaths

21

Original post by Anon736

For a question like this how do you know which one to use?

cosA is positive, therefore it has to be in the 4th. Remember the diagram

Reply 7

Anon736

OP

8

Original post by B_9710

Reflex angle covers both 3rd and 4th quadrant since refle angle is angle bigger than π. So this tells you that in fact angle A is in fact in the 4th quadrant. So sinA will be negative and so will tanA - just sketch the graphs and you will see this.