侧面积 = (a + b)*斜高/2 * 4 = aa + bb=>斜高 = (aa+bb)/2(a+b)斜高，高，(b-a)/2,构成直角三角形=>高^2 = (((aa+bb)/2(a+b))^2 - ((b-a)/2)^2)=>高=√2*ab/(a+b)
