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In any triangle ABC, the cosine rule states that (3.1) <math display='block'> <mrow> <msup> <mi>c</mi> <mn>2</mn> </msup> <mo>=</mo><msup> <mi>a</mi> <mn>2</mn> </msup> <mo>+</mo><msup> <mi>b</mi> <mn>2</mn> </msup> <mo>−</mo><mn>2</mn><mi>a</mi><mi>b</mi><mi>cos</mi><mi>θ</mi><mo>,</mo> </mrow> </math> $${{c}^{2}}={{a}^{2}}+{{b}^{2}}-2ab\cos \theta ,$$ where a, b, c are the lengths of the sides BC, CA, AB respectively and θ is the angle ACB. If a, b, c are vectors represented by the displacements <math display='block'> <mrow> <mover accent='true'> <mrow> <mi>C</mi><mi>B</mi> </mrow> <mo stretchy='true'>→</mo> </mover> <mo>,</mo><mover accent='true'> <mrow> <mi>C</mi><mi>A</mi> </mrow> <mo stretchy='true'>→</mo> </mover> <mo>,</mo><mover accent='true'> <mrow> <mi>A</mi><mi>B</mi> </mrow> <mo stretchy='true'>→</mo> </mover> </mrow> </math> $$\overrightarrow{{CB}},\overrightarrow{{CA}},\overrightarrow{{AB}}$$ respectively, equation (3.1) may be written in the form <math display='block'> <mrow> <msup> <mrow> <mrow><mo>|</mo> <mi>c</mi> <mo>|</mo></mrow> </mrow> <mn>2</mn> </msup> <mo>=</mo><msup> <mrow> <mrow><mo>|</mo> <mi>a</mi> <mo>|</mo></mrow> </mrow> <mn>2</mn> </msup> <mo>+</mo><msup> <mrow> <mrow><mo>|</mo> <mi>b</mi> <mo>|</mo></mrow> </mrow> <mn>2</mn> </msup> <mo>−</mo><mn>2</mn><mrow><mo>|</mo> <mi>a</mi> <mo>|</mo></mrow><mrow><mo>|</mo> <mi>b</mi> <mo>|</mo></mrow><mi>cos</mi><mi>θ</mi><mo>,</mo> </mrow> </math> $${{\left| c \right|}^{2}}={{\left| a \right|}^{2}}+{{\left| b \right|}^{2}}-2\left| a \right|\left| b \right|\cos \theta ,$$ where θ is the angle between the vectors a and b.