15)a+b+c=90 ,a=9b+c=81 ,c^2=a^2+b^2b^2-c^2=a^2(b+c)(c-b)=a^2=81c-b=1b+c=81 c=41斜边=4116)因三角形AOB是等边三角形AB=OA=1/2AC=1/2*6=3BC^2=AC^2-AB^2=6^2-3^2=27BC=3√3矩形周长=2(3+3√3)=6(1+√3)
