sum of interior angle of a triangle(三角形):180°
sum of interior angle of a quadrilateral(四边形):360°
a+b+c+y=360°
d+e+x=180°
f+g+h+z=360°
x+y+z=180°
a+b+c+d+e+f+g+h+x+y+z=360°+360°+180°
=900°
a+b+c+d+e+f+g+h=900°-(x+y+z)
=900°-180°
=720°
对不起有点乱(。ノω\。)