solve by using law of cosines (i keep getting it wrong)

[sportsstar469]

ok i msupposed to solve this by either using the law of sines or the law of cosines since it is an SSS triangle i am using the law of cosines.
a=17 meters b = 20 meters c=31 meters.
i did this exactly like thye book sas and i keep getting it wrong.
first step was i wanted to solve for angle C.
i wrote the forumla c^2=a^2+b^2-2(a)(b) times the cos of c
i then wrote it as cos of c=a^2 +b^2-c^2 divided by 2ac.

so after plugging in the values i got cos(c)=17^2+20^2-31^2 all over 680

the correct answer is c equals 113.6.

so idkj what i did wrong!!!!!!!!!

 

[sportsstar469]

i have a test tomorrow, and i only learned this stuff on tusday so im very scared, because not only do i not know this but the test is on two more chapters. my last thread got no replies sso if someone can help me thatd be great. i get a lot of weird responses fro mpeople on here but i know you guyys are good at math so please help me.

 

[Deleted member 4993]

ok i msupposed to solve this by either using the law of sines or the law of cosines since it is an SSS triangle i am using the law of cosines.
a=17 meters b = 20 meters c=31 meters.
i did this exactly like thye book sas and i keep getting it wrong.
first step was i wanted to solve for angle C.
i wrote the forumla c^2=a^2+b^2-2(a)(b) times the cos of c
i then wrote it as cos of c=a^2 +b^2-c^2 divided by 2ab.

so after plugging in the values i got cos(c)=17^2+20^2-31^2 all over 680 <<< This is correct

and I do get:

cos(C) = -0.4

and

C = 113.5781785 = 113.6°

So where is the problem??? Why do you think you did anything wrong??

the correct answer is c equals 113.6.

so idkj what i did wrong!!!!!!!!!

 

[sportsstar469]

i think it was just a calculator fail. for some reason the calculator kept giving me an error, but i cleared its memory and it worked again. thanks for the response sabbasgh kahs.