Week 17- Cos Law

This week, I have learned how to find the missing angle or missing length by using cos law.

There are 2 ways to use the cos law.

  1. To find the missing angle
  2. To find the missing lenght

To find the missing angle, we use : cosC= a^2+b^2-c^2 \div 2ab

To find the missing length, we use : c^2=a^2+b^2-2ab cos C

Let’s try an example

Image result for cos law examples

We know that angle C is 37, and we are looking for the length C.

Lets write down every thing in the formula.


c^2=a^2+b^2-2ab cos C

c^2=11^2+8^2-2(11)(8) cos 37

C= 3.9 cm

Lets try another example.

Image result for cos law examples


cosC= a^2+b^2-c^2 \div 2ab

cos X= 6^2+5^2-8^2\div2(6)(5)

X= 6^2+5^2-8^2\div2(6)(5) * cos^-1


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