ta có:
2abc(cosA+cosB) = $2abc(\frac{b^2+c^2-a^2}{a^2+c^2-b^2})$= 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) = $2abc(\frac{b^2+c^2-a^2}{a^2+c^2-b^2})$= 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) = $2abc(\frac{b^2+c^2-a^2}{a^2+c^2-b^2})$= ab2 +
ba2 + ac2 + bc2 - a3 - b3
= (a + b) (c2 – (a + b)2 = (a + b)(c + b – a)(c + a – b) (đpcm).