三數和的完全平方公式

What is the expanded form of (a + b + c)²?

• a² + b² + c² + 2ab + 2bc + 2ca

• a² + b² + c² + ab + bc + ca

• a² + b² + c² + 2ab + 2bc + ca

• a² + b² + c² + 2ab + bc + 2ca

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Expand (x - y - 3z)².

• x² + y² + 9z² - 2xy + 6yz - 6zx

• x² + y² + 3z² - 2xy - 6yz - 6zx

• x² + y² + 9z² + 2xy - 6yz + 6zx

• x² + y² + 9z² - 2xy - 6yz + 6zx

Given a² + b² + c² = 8 and ab - bc - ca = 4, what is the value of (a + b - c)²?

• 12

• 16

• 20

• 24

If a² + b² + c² = 8 and ab - bc - ca = 4, what is the value of a + b - c?

• 4

• -4

• ±4

• 16