Consider a triangle where a=3, b=5 and c=7. Use the law of cosines to find angle C.
possible answers are 60 deg, 120 deg, 135 deg or not possible.
I came up with 3.97 so I must have did this wrong. Pls help

Question

Consider a triangle where a=3, b=5 and c=7. Use the law of cosines to find angle C. 
possible answers are 60 deg, 120 deg, 135 deg or not possible. 
I came up with 3.97 so I must have did this wrong. Pls help

anonymous · Answer

As long as you are consistent you will be fine so 
$$c ^{2}=a^{2}+b^{2}-2abCos(C)$$
$$7^{2}=3^{2}+5^{2}-2*3*5*Cos(C)$$
$$49=9+25-30*Cos(C)$$
$$15=-30*Cos(C)$$
$$-1/2=Cos(C)$$
$$Cos^{-1}(-1/2)=C$$
So, C=120.

anonymous · Answer

Oh, thank you so much! I was lost on this. For some reason your response didn't show up in notifications and i just clicked on my question and your response was there. Am I suppose to refresh my screen often?