# Thread: The Cosine Rule

1. ## The Cosine Rule

Hi guys,

I have a question on the cosine rule that i think i am getting right but the books answers tell me that its wrong. Its really easy:

In triangle ABC, a=8cm, b=10cm, C(Angle)=56.2 degrees. Find c(missing side).

Thanks in advance (im sure im right)

Steve

2. well it isnt that difficult o.o" books aren't perfect xD

C^2 = A^2 + B^2 - 2AB Cos(c)
C^2 = 8^2 + 10^2 - 2(8)(10) Cos (56.2)
C = 8.65983265

3. Thanx heaps, but thats exactly what i did and i keep ending up with around 14.

So i got:

a^2=100 + 64 -(-52.8)
a^2=164+52.8
a=14.724......

4. i think you got the triangle wrong o.o"

you got the 100 and 64 correctly but your last term i don't understand how you get it

try :
Cosine Rule

5. I did:

2x8x10xcosx56.2

6. If you are using a calculator calculate the value for cos 56.2 (in degree) first before you multiply it with 2, 8 and 10

well if you still don't get

cos 56.2 = 0.556295615

then it is time you change a new calculator o.o" i tried on two of mine and they obtained the same value

7. Ok so i get 89.007298......and i add it to 164 and get 253.007....and then i square-root it and get 15.906.....

8. you need to minus 89.007298 from 164 not adding them up.

C^2 = A^2 + B^2 - 2A(B)cos (c)
= 164 - 89.007298
= 74.99270152

C = sqrt(74.99270152)
= 8.65983265

9. wow thankyou so much, it makes sense but why do you have to multiply cos first?

10. Oh its ok i got the answer now. I kept getting 14 but it was around 8

11. well sometimes when you press in the numbers in the calculator it misreads it. therefore it is best you do the most complicated part first as one single calculation which is the cosine part.