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# Maths C2 Sine/Cosine help please watch

1. The area of a triangle is 10cm^2. the angle between two of the sides, of length 6 cm and 8 cm respectively, is obtuse. Work out the size of this angle.

So I did: Area = 0.5 x a x b x sinC
10 = 0.5 x 8 x 6 x sinC
10 = 24 sinC
sinC = 0.416666666
c = 24.62431831

But this isn't obtuse and the answer says 155, any help as to where I have gone wrong?
2. (Original post by dav22)
The area of a triangle is 10cm^2. the angle between two of the sides, of length 6 cm and 8 cm respectively, is obtuse. Work out the size of this angle.

So I did: Area = 0.5 x a x b x sinC
10 = 0.5 x 8 x 6 x sinC
10 = 24 sinC
sinC = 0.416666666
c = 24.62431831

But this isn't obtuse and the answer says 155, any help as to where I have gone wrong?
Your calculator will give you the acute angle with that sine value. to find the one you're after use 180 -(angle from calc.)
3. (Original post by joostan)
Your calculator will give you the acute angle with that sine value. to find the one you're after use 180 -(angle from calc.)
How did I not notice that! Hahahah thank you!!
4. (Original post by dav22)
How did I not notice that! Hahahah thank you!!
np
5. Yeah, always remember in the back of your mind that sin(x)=sin(180-x) ^^
6. (Original post by aznkid66)
Yeah, always remember in the back of your mind that sin(x)=sin(180-x) ^^
Unless it's negative.
7. (Original post by L'Evil Fish)
Unless it's negative.
What do you mean?
8. (Original post by L'Evil Fish)
Unless it's negative.
...Hm? isn't -theta theta degrees below the positive x-axis and 180-theta theta degrees below the negative x-axis, i.e., don't they share the same y value?
9. (Original post by L'Evil Fish)
Unless it's negative.
sin(-30) does equal sin(210) . . .
10. (Original post by metaltron)
What do you mean?
(Original post by aznkid66)
...Hm? isn't -theta theta degrees below the positive x-axis and 180-theta theta degrees below the negative x-axis, i.e., don't they share the same y value?
Ignore me
11. (Original post by joostan)
sin(-30) does equal sin(210) . . .
Sorry, ignore me.

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