equation of line passing throught MN
(y-2)/(x-1) = (a-2)/(3-1)
2(y-2) = (a-2)(x-1)
y = [(a-2)/2] x + (2- a)/2
斜率 = (a-2)/2
if (a-2)/2 >0
a> 2 ,倾斜角是锐角
a