cosB = (a+b)(b+c-a)(c+a-b) / 2abc
cosB = (a + b).(c² - a² - b² + 2ab)/(2abc)
cosB = (a + b).(2ab - 2ab.cosC)/(2abc)
cosB = (a + b).(1 - cosC)/c
cosB = (2RsinA + 2RsinB)(1 - cosC)/2RsinC
cosB = (sinA + sinB)(1 - cosC)/sinC
<=> cosB.sinC = sinA + sinB - sinA.cosC - sinB.cosC
<=> sinB.cosC + cosB.sinC + sinA.cosC - sinA - sinB = 0
<=> sin(B + C) + sinA.cosC - sinA - sin(A + C) = 0
<=> sinA.cosC - sinA.cosC - cosA.sinC = 0
<=> cosA.sinC = 0
<=> cosA = 0 hoặc sinC = 0
<=> A = 90 độ