How to calculate B = root A^2 + A^2 + 2AA Cos theta?

This is done by replacing α with θ/2: sin(θ) = sin(2(θ/2)) = 2 sin(θ/2) cos(θ/2).

Alternatively, you could say this: Let θ = 2α; then sin(θ) = sin(2α) = 2 sin(α)cos(α) = 2 sin(θ/2) cos(θ/2) because α = θ/2.

Such substitutions are common throughout trigonometry and especially in calculus, so it is important to become familiar with this process.
I got it now. Thank you all for your kind efforts. Thanks a lot.
 
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