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Maths C2 Sine/Cosine help please

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? :smile:
Reply 1
Avatar for joostan
joostan
13
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? :smile:

Your calculator will give you the acute angle with that sine value. to find the one you're after use 180 -(angle from calc.) :smile:
Reply 2
Avatar for dav22
dav22
OP
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.) :smile:


How did I not notice that! Hahahah thank you!!
Reply 3
Avatar for joostan
joostan
13
Original post by dav22
How did I not notice that! Hahahah thank you!!


np :biggrin:
Reply 4
Avatar for aznkid66
aznkid66
1
Yeah, always remember in the back of your mind that sin(x)=sin(180-x) ^^
Original post by aznkid66
Yeah, always remember in the back of your mind that sin(x)=sin(180-x) ^^


Unless it's negative.
Reply 6
Avatar for metaltron
metaltron
11
Original post by L'Evil Fish
Unless it's negative.


What do you mean?
Reply 7
Avatar for aznkid66
aznkid66
1
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?
Reply 8
Avatar for joostan
joostan
13
Original post by L'Evil Fish
Unless it's negative.


sin(-30) does equal sin(210) . . .
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 :tongue:
Original post by joostan
sin(-30) does equal sin(210) . . .


Sorry, ignore me.

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