Can someone please explain why you use c^2=a^+b^2-2abcosC formula for this problem. How do you determine to use this formula as opposed to the other two law of cosines formulas?

#### Top Answer

Length of side c is 6.478... View the full answer

- Angle A can also be determined from cosine rule, cosA =(b^2+c^2-a^2)/2bc
- Rahul.royaltyger
- Sep 02, 2018 at 12:50pm

- Can you explain more on the step for c 2 =25+24× √ 2 1 =41.97056. And your final answer is 6.478?
- jonpaulwilson001
- Sep 02, 2018 at 4:24pm

- Actually cos (135)=cos(90+45)=-cos(45)=1/√2 ...since 135 ° is in 2nd quadrant ..and in 2nd quadrant cosine is negative.
- Rahul.royaltyger
- Sep 02, 2018 at 5:02pm

- C2= 4^2 + 3^2 -2(4)(3)cos 135 = 16 + 9 -(-24/√2) = 41.97
- Rahul.royaltyger
- Sep 02, 2018 at 5:04pm

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#### Other Answers

Since we have two sides and the included angle, use the... View the full answer

The length of the... View the full answer

- So we use cos(C) = 2ab a 2 +b 2 −c 2 because we are trying to find C ?
- jonpaulwilson001
- Sep 02, 2018 at 4:04pm

- Yes, if there is a need to find the length of side c of triangle, we use the formula cos(C). If there is a need to find the length of side a of triangle, we use the formula cos(A). If there is a need to find the length of side b of triangle, we use the formula cos(B). or vice versa
- ankitmittal
- Sep 02, 2018 at 4:10pm

- Ok thank you you have been very helpful, but ur final answer is wrong
- jonpaulwilson001
- Sep 02, 2018 at 4:15pm

- Yes, i have checked it was c^2 = 41.97 and correct ans is c = 6.48, sorry for that
- ankitmittal
- Sep 02, 2018 at 4:25pm

- how do you get 6.48 as the final answer please explain?
- jonpaulwilson001
- Sep 02, 2018 at 4:31pm

- see the solution there was only one mistake of cos(135), the correct is cos(135) = -(sqrt{2}/2), except that every thing is correct in the solution. In the comment box it is not possible to write the mathematical terms.
- ankitmittal
- Sep 02, 2018 at 4:35pm

Hi, student! I'm glad to help you.... View the full answer