sin240=-sin30???

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?

Reply 1

joostan

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. :tongue:

check your answers.
The way you structured the question could also be clearer.

(edited 11 years ago)

Reply 2

SophieL1996

Original post by joostan

You need a better book. :tongue:

are my answers correct though?

Reply 3

Indeterminate

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?

\sin(240) = -\sin(60)

\sin(-x) = -\sin(x)

Reply 4

Mr M

Study Forum Helper

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?

Reply 5

SophieL1996

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..

(edited 11 years ago)

Reply 6

davros

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!

Reply 7

Mr M

Study Forum Helper

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..

\sin(180+60)=\sin180\cos60+\cos180\sin60

Reply 8

SophieL1996

I think the acute angle has to be made with the x-axis always, this is where I am wrong :smile:

Reply 9

SophieL1996

Original post by Mr M

\sin(180+60)=\sin180\cos60+\cos180\sin60

I do not understand that, but I know my mistakes

Reply 10

joostan

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.

(edited 11 years ago)

Reply 11

davros

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

Reply 12

SophieL1996

Original post by joostan

You don't need a better book. :tongue:

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 :tongue:

Reply 13

joostan

Original post by SophieL1996

Hey! you were just agreeing with me saying the book is wrong! Now you've changed you mind :tongue:

Yes, I paired each of the book's values with a different value given.
A foolish error, but an easy one to make. :s-smilie:

This is because the presentation of the question was a little confusing.

Reply 14

Pastaferian

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?