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# sin240=-sin30??? Watch

1. the book says it =-sin60 but I think that's wrong? and also sin(-80)=-sin10 but the book says it = -sin80 and sin(-200)=sin70 but the book says it =sin20 ? ? ? any ideas?
2. (Original post by SophieL1996)
the book says it =-sin60 but I think that's wrong? and also sin(-80)=-sin10 but the book says it = -sin80 and sin(-200)=sin70 but the book says it =sin20 ? ? ? any ideas?
You don't need a better book. check your answers.
The way you structured the question could also be clearer.
3. (Original post by joostan)
You need a better book.
are my answers correct though?
4. (Original post by SophieL1996)
the book says it =-sin60 but I think that's wrong? and also sin(-80)=-sin10 but the book says it = -sin80 and sin(-200)=sin70 but the book says it =sin20 ? ? ? any ideas?

5. (Original post by SophieL1996)
the book says it =-sin60 but I think that's wrong? and also sin(-80)=-sin10 but the book says it = -sin80 and sin(-200)=sin70 but the book says it =sin20 ? ? ? any ideas?
Surely you just need to type these values into your calculator to just answer your own question?
6. (Original post by Mr M)
Surely you just need to type these values into your calculator to just answer your own question?
Ah yes. Well in that case I appear to be wrong. But why does sin240= -sin60? I don't see where the 60 is from..
7. (Original post by SophieL1996)
the book says it =-sin60 but I think that's wrong? and also sin(-80)=-sin10 but the book says it = -sin80 and sin(-200)=sin70 but the book says it =sin20 ? ? ? any ideas?
Not quite sure which statements you are claiming to be true, but there are 2 basic rules you should know:
sin(-x) = -sin(x)

sin(180-x) = sin x

so sin 240 = sin(180-240) = sin(-60) = -sin 60
sin(-200) = -sin(200) = -sin(180-200) = -sin(-20) = sin 20

You can always check with a calculator if unsure - remember to set in Degrees for these values!
8. (Original post by SophieL1996)
Ah yes. Well in that case I appear to be wrong. But why does sin240= -sin60? I don't see where the 60 is from..
9. I think the acute angle has to be made with the x-axis always, this is where I am wrong
10. (Original post by Mr M)
I do not understand that, but I know my mistakes
11. (Original post by SophieL1996)
I do not understand that, but I know my mistakes
This is a double angle formula in C3, though it will appear in your formula book.
12. (Original post by SophieL1996)
I do not understand that, but I know my mistakes
MrM was giving you a specific instance of the addition rule:
sin(A+B) = sinAcosB + cosAsinB
13. (Original post by joostan)
You don't need a better book. check your answers.
The way you structured the question could also be clearer.
Hey! you were just agreeing with me saying the book is wrong! Now you've changed you mind
14. (Original post by SophieL1996)
Hey! you were just agreeing with me saying the book is wrong! Now you've changed you mind
Yes, I paired each of the book's values with a different value given.
A foolish error, but an easy one to make.
This is because the presentation of the question was a little confusing.
15. (Original post by SophieL1996)
the book says it =-sin60 but I think that's wrong? and also sin(-80)=-sin10 but the book says it = -sin80 and sin(-200)=sin70 but the book says it =sin20 ? ? ? any ideas?
The book is correct. The following is true for all values of X...

sin X = sin (180-X) = - sin (180+X) = - sin (360-X) = sin (360+X) = ...
16. (Original post by Mr M)
Surely you just need to type these values into your calculator to just answer your own question?
^ Clever guy

I wanted to 'like' your comment but i've run out of likes today. So here's a comment instead!

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