# finding the measure of an angle

#### starrynight

if a triangle has the sides a=412, b=324, c=636, what is the measure of angle A?

we can choose from these answers:

a) 119.1
b) 34.5
c) 48.7
d) 26.4
e) none of these

#### earboth

MHF Hall of Honor
if a triangle has the sides a=412, b=324, c=636, what is the measure of angle A?

we can choose from these answers:

a) 119.1
b) 34.5
c) 48.7
d) 26.4
e) none of these
1. Draw a sketch.

2. Use the Cosine rule. And then show your work. We'll check if everything is OK.

3
It's b)

#### masters

MHF Helper
if a triangle has the sides a=412, b=324, c=636, what is the measure of angle A?

we can choose from these answers:

a) 119.1
b) 34.5
c) 48.7
d) 26.4
e) none of these
Hi starrynight,

Use the law of cosines: $$\displaystyle a^2=b^2+c^2-2bc \cos A$$

• starrynight

#### starrynight

i get an answer of 145.5202602. is this correct?

#### earboth

MHF Hall of Honor
i get an answer of 145.5202602. is this correct?
Obviously not:

1. According to the Cosine rule an angle greater than 90° must be opposite the largest side in the triangle. That is the side c and not the side a, which is opposite the angle at A.

2. You probably have forgotten about the negative sign in the denominator and therefore the correct result would be:

180° - 145.5202602° = 34.479...°