|a+b|² = (a+b)² = a² + 2(a·b) + b²

• a² = |a|² = (6·6 + 8·8) = 36+64 = 100, тогда |a| = 10
• b² = |b|² = 1
• 2(a·b) = 2·|a|·|b|·cos(a, b) = 2·10·1·cos 60° = 20·1/2 = 10

|a+b|² = 100 + 10 + 1 = 111

|a+b| = √111