ta có:
2abc(cosA+cosB) = 2abc(b2+c2−a2a2+c2−b2)= ab2 +
ba2 + ac2 + bc2 - a3 - b3
= (a + b) (c2 – (a + b)2 = (a + b)(c + b – a)(c + a – b) (đpcm).
ta có:
2abc(cosA+cosB) =
2abc(b2+c2−a2a2+c2−b2)= ab2 +
ba2 + ac2 + bc2 - a3 - b3
= (a + b) (c2 – (a + b)2 = (a + b)(c + b – a)(c + a – b) (đpcm).