Ta có:
` B = \frac{1}{1 + a + ab} + \frac{1}{1 + b + bc} + \frac{1}{1 + c + ca} `
` <=> B = \frac{1}{1 + a + ab} + \frac{abc}{abc + b + bc} + \frac{abc}{abc + c + ca} `
` <=> B = \frac{1}{1 + a + ab} + \frac{ac}{ac + abc + c} + \frac{ab}{ab + 1 + a} `
` <=> B = \frac{1}{1 + a + ab} + \frac{a}{a + ab + 1} + \frac{ab}{ab + 1 + a} `
` <=> B = \frac{1 + a + ab}{1 + a + ab} `
` <=> B = 1 `
Vậy ` B = 1 `