m = (x₁,y₁) = (b • cosC,-1)
n = (x₂,y₂) = ((c - 3a) • cosB,1)
∵m和n共线,所以
x₁/x₂= y₁/y₂
x₁y₂- x₂y₁= 0
(b • cosC)(1) - ((c - 3a) • cosB)(-1) = 0
b • cosC + (c - 3a) • cosB = 0,运用正弦定理：a/sinA = b/sinB = c/sinC = k
k • sinB • cosC + (k • sinC - 3 • k • sinA) • cosB = 0
sinBcosC + cosBsinC - 3sinAcosB
sin(B + C) = 3sinAcosB
sin(180° - A) = sinA = 3sinAcosB
=> cosB = 1/3
=> sinB = √(3² - 1)/3 = (2√2)/3