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# Example 1. 3 A triangle [Fig 1.15(a)] has sida of 3 ft and 4 ftand the included angle equal to 135. Find the third '. and the angle it forms with...

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?

Example 1. 3 A triangle [Fig 1.15(a)] has sida of 3 ft and 4 ftand the included angle equal to 135°. Find the third '.
and the angle it forms with line AE. &quot; Scanned by CamScanner CHAPTER ONE

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

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

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