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Question 5

Now |a - b|^2 = 1 + 1 - 2 cos (180 - C)

And |b - c|^2 = 1 + 1 - 2 cos (180 - A)

Also |c - a|^2 = 1 + 1 - 2 cos (180 - B)

Now adding we have 6 + 2( cos A + cos B + cos C)

As A, B and C are to be 60 degree, we have 6 + 2 (3/2) = 9 (not less than 9)

Diagram would help you a lot

All the best

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