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R² = a² + b² + 2 a b cos θ
R = √ ( a² + b² + 2 a b cos θ )
R = √ ( 3² + 4² + 2 ∙ 3 ∙ 4 ∙ cos 45° )
R = √ ( 9 + 16 + 24 ∙ cos 45° )
R = √ ( 25 + 2 ∙ 12 √2 / 2 )
R = √ ( 25 + 12 √2 )
R = 6.4784691671