Maths-Cosine rule

Hope this helps :smile:

Reply 2

OP

Original post by LeFeuilly

Hope this helps :smile:

Hope what helps?
its just a square box? :tongue:

Reply 3

Original post by RBalboa

Hope what helps?
its just a square box? :tongue:

Can you not see the image?

Reply 4

1777481

10

Find the length of BC.

AnswerDid you get 4.86cm? If so, well done.
If not, remember to use the formula:
a² = b² + c² - 2bc cos A
Substitute the values into the formula.
a² = 7² + 3² - 2 × 3 × 7 cos 35
a² = 58 - 42 cos 35
a² = 23.5956
a = 4.86cm

Reply 5

OP

Original post by LeFeuilly

Can you not see the image?

No not really PM me

Reply 6

OP

Original post by AyshaU

Find the length of BC.

AnswerDid you get 4.86cm? If so, well done.
If not, remember to use the formula:
a² = b² + c² - 2bc cos A
Substitute the values into the formula.
a² = 7² + 3² - 2 × 3 × 7 cos 35
a² = 58 - 42 cos 35
a² = 23.5956
a = 4.86cm

It need to calculate the angle of ABC but all the question gives me is a length of 8.1cm 7.5cm and the angle ACB 30degrees

Reply 7

OP

http://imgur.com/xrLTp7J
use this, it's my question

Reply 8

Original post by RBalboa

No not really PM me

Ehhh I'll just type it up:

82°
/\
5cm (c)/ \ 6cm (b)
/____ \
a cm

Find "a" correct to 3 s.f.

a²=b²+c²-2bc cosA
a=√6²+5² -2 (6)(5) cos82
a= 7.26 cm

Reply 9

Smaug123

Original post by RBalboa

How do you do the cosine rule?
I know the equation, but please can someone give me an example of a question and show me the method?
Thank you

It's basically just Pythagoras. If you take a non-rightangled triangle and drop a perpendicular from one vertex to get two right-angled triangles, and apply Pythagoras to one of them and use the definitions of cos and sin on the other one, you get the cosine rule. There's no magic about it; you just use it in the same kinds of ways you'd use Pythagoras.

Reply 10

OP

Original post by LeFeuilly

Ehhh I'll just type it up:

82°
/\
5cm (c)/ \ 6cm (b)
/____ \
a cm

Find "a" correct to 3 s.f.

a²=b²+c²-2bc cosA
a=√6²+5² -2 (6)(5) cos82
a= 7.26 cm

Look above at the link I sent that's what my question looks like

Reply 11

OP

Original post by Smaug123

It's basically just Pythagoras. If you take a non-rightangled triangle and drop a perpendicular from one vertex to get two right-angled triangles, and apply Pythagoras to one of them and use the definitions of cos and sin on the other one, you get the cosine rule. There's no magic about it; you just use it in the same kinds of ways you'd use Pythagoras.

err okay I think

Reply 12

Original post by LeFeuilly

Ehhh I'll just type it up:

82°
/\
5cm (c)/ \ 6cm (b)
/____ \
a cm

Find "a" correct to 3 s.f.

a²=b²+c²-2bc cosA
a=√6²+5² -2 (6)(5) cos82
a= 7.26 cm

If you're using EDEXCEL iGCSE Mathematics A book 2, it's Exercise 87 question one (and sorry about the whole image fiasco - I can't send anything by mail -_-")

Reply 13

OP

Original post by LeFeuilly

If you're using EDEXCEL iGCSE Mathematics A book 2, it's Exercise 87 question one (and sorry about the whole image fiasco - I can't send anything by mail -_-")

I'm not but if u click the link I sent you then my question will come up :smile:

Reply 14

Original post by RBalboa

I'm not but if u click the link I sent you then my question will come up :smile:

So, like the people above me have said, cosine rule is basically just substitution + pythagoras. In regards to your question, you need to use the SINE rule to answer part a. As for part b, you need 1/2 ab sin c to solve that.

Hope that helps :smile:

Reply 15

iMacJack

16

Original post by RBalboa

How do you do the cosine rule?
I know the equation, but please can someone give me an example of a question and show me the method?
Thank you

You can use the cosine rule predominantly when you're given two sides and an included angle (an included angle means an angle which is subtended between the two lengths.)

Reply 16

OP