MYTUTOR SUBJECT ANSWERS

489 views

When do I use a cosine rule over a sine rule?

Both the sine and the consine rule applies to any triangle, you do not necessary need a right angle! 

You can usually use the cosine rule when you are given two sides and the included angle (SAS) or when you are given three sides and want to work out an angle (SSS)

In order to use the sine rule, you need to know either two angles and a side (ASA) or two sides and a non-included angle (SSA).

- Use the sine rule when a problem involves two sides and two angles
- Use the cosine rule when a problem involves three sides and one angle

The cosine equation:
a= b2 + c2 - 2bccos (A)
(version given on formula sheet to find missing side)
cos (A) = (b+ c2 - a2) / 2bc
(use this version for finding a missing angle)

The sine equation:
a / sinA = b / sinB = c / sinC (use to find the missing side)
sinA / a = sinB / b = sinC / c (use to find the missing angle)

Key points to remember:

Sine Rule:
- As you label the triangle, each side has the same letter as its opposite angle!
- If you are wanting to find the angle in the equation remember to use the sin-1 function on your calculator.
- The greater the length is the greater is the opposite angle!

Cosine Rule:
- Remeber to use BIDMAS when you work out the value on the right hand side for finding the missing side, hence put brackets around the (2bccos (A) ).

Jessica F. A Level Economics tutor, A Level Maths tutor, GCSE Maths t...

2 years ago

Answered by Jessica, a GCSE Maths tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist

328 SUBJECT SPECIALISTS

£22 /hr

Josh R.

Degree: Mathematics (Masters) - Warwick University

Subjects offered: Maths, Further Mathematics

Maths
Further Mathematics

“I study Maths at Warwick, I hope to be able to teach your child to love Maths and how to achieve top exam grades. ”

£18 /hr

James B.

Degree: Mathematics and Economics (Bachelors) - Nottingham University

Subjects offered: Maths, Economics

Maths
Economics

“About me : I am currently studying Maths and Economics at Nottingham University, about to go into my second year. Over the summer I have been in America teaching and counselling kids at a summer camp and I hope to bring what I have le...”

£18 /hr

Ewan H.

Degree: Aerospace Engineering (Masters) - Bristol University

Subjects offered: Maths, Physics+ 1 more

Maths
Physics
-Personal Statements-

“About me:I am currently a first year at Bristol University studying Aerospace Engineering. Given my degree, my interests have always been interested in thesciences and maths.I have always been involved in various tutoring groups, m...”

About the author

£18 /hr

Jessica F.

Degree: Economics (Bachelors) - Exeter University

Subjects offered: Maths, Economics

Maths
Economics

“Top tutor from the renowned Russell university group, ready to help you improve your grades.”

MyTutor guarantee

You may also like...

Posts by Jessica

Explain why monopolies may be an undesirable form of market structure

Finding stationary points

When do I use a cosine rule over a sine rule?

Why have inequalities increased in recent years?

Other GCSE Maths questions

Solve this quadractic equation: x^2 - 8x + 15 = 0

How to find conditional probability

Solve the simultaneous equations 5x+2y=11 and x-y=-2.

Pythagoras' Theorem

View GCSE Maths tutors

Cookies:

We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss

mtw:mercury1:status:ok