2A = (A – B)2 + (B – C)2 + (C – A)2.TA CĨ

261. 2A = (a – b)

²

+ (b – c)

²

+ (c – a)

²

.

Ta cĩ : c – a = - (a – c) = - [(a – b) + (b – c)] = - (

2

^{+ 1 +}

2

- 1) = - 2

2

^.

Do đĩ : 2A = (

2

^{+ 1)}

²

^{+ (}

2

^{- 1)}

²

^{+ (-2}

2

⁾

²

= 14. Suy ra A = 7.