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# need help with triangles trig Watch

1. A triangle has 3 sides. 4.2, 5.3, 7.6. Calculate the size of the largest angle of the triangle (1 d.p) I know that the Cosine rule is used but I don't know how to work it out. The formula I used to try and solve this was "Cos C=a (squared)+b(squared) - c (squared) then divided by 2ab in order to find the angle. Please could someone help me because I've seen people do the cosine formula for side. Please could someone help me ASAP. Thank you
2. In every triangle the largest side is opposite the largest angle. In this case the largest angle is the angle opposite the side with length 7.6.

7.62 = 4.22+5.32 – 2 x 4.2 x 5.3 x Cos(A)

Cos(A) = (4.22+5.32 _ 7.62) / 2 x 4.2 x 5.3
Cos(A) = - 0.27…

A = Cos-1 -0.27…= 105.68
3. (Original post by IXxi HIIDA ixXI)
In every triangle the largest side is opposite the largest angle. In this case the largest angle is the angle opposite the side with length 7.6.

7.62 = 4.22+5.32 – 2 x 4.2 x 5.3 x Cos(A)

Cos(A) = (4.22+5.32 _ 7.62) / 2 x 4.2 x 5.3
Cos(A) = - 0.27…

A = Cos-1 -0.27…= 105.68

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