Question

In triangle ABC, a=3,b=5,c=7, find the measure of angle c?

Answer 1

i have all three sides, so there is no need for the pythagorean theorem so where do i got from there?

Answer 2

use the law of sines

Answer 3

im not familiar

Answer 4

sorry.., i mean use the law of cosines

Answer 5

how do you know which one to use, and what are they?

Answer 6

C^2 = A^2 + B^2 + 2AB cos c

Answer 7

please try.., and i'll see

Answer 8

120?

Answer 9

what did u get it?

Answer 10

120

Answer 11

am i right?

Answer 12

good joob! ur right!

Answer 13

great, can you just explain why you chose law of cos over law of sin?

Answer 14

sorry..,the right answer is not 120

Answer 15

i dont understand...i did it right :(

Answer 16

-1/2=cos C=120

Answer 17

C^2 = A^2 + B^2 + 2AB cos c
7^2 = 3^2 + 5^2 + (2x 3x 5 ) cos c
49 = 9 + 25 + 30 cos c
49 = 34 + 30 cos c
49 - 34 = 30 cos c
15 = 30 cos c
1 = 2 cos c
1/2 = cos c

am i wrong?

Answer 18

ur formula is wrong, its -2AB cos c

Answer 19

oh yes.., i'm wrong, i'm sorry..,

Answer 20

ur relly good job.., the answer is 120

Answer 21

lol ok well.. why do we use law of cos and not the law of sin??

Answer 22

is there a law of sin? not sure

Answer 23

oh yes, there is a law of sin, so do you know why?

Answer 24

ok i'll explain..,
If there is no angle is known, it will definitely use the law of cosines

Answer 25

what if there is one known angle?

Answer 26

if one known angle and all of side is known, it will definitely use the law of sines

Answer 27

ok, are you positive, i have an exam tomorrow, im taking your word for it lol, i hope your right, thanks again :)

Answer 28

ur welcome..., good luck for ur exam..,

Answer 29

please tell me the result of ur exam, ok :)

Answer 30

thanks! and i will!

Answer 31

@Life
there is reference for u
please read it deeply
http://www.mathwarehouse.com/trigonometry/law-of-sines-and-cosines.php

Answer 32

wow, thank you so much, this is great!

Answer 33

ur welcome @Life