两式相减
16x+12y-40=0
4x-3y-10=0
这就是公共弦所在直线
y=(4x-10)/3代入x^2+y^2-10x-10y=0并整理
5x^2-58x+80=0
x1+x2=58/5,x1x2=16
(x1-x2)^2=(x1+x2)^2-4x1x2=1764/25
y1-y2=[(4x1-10)/3]-[(4x2-10)/3]=4(x1-x2)/3
(y1-y2)^2=16(x1-x2)^2/9=3136/25
所以弦长=√[(x1-x2)^2+(y1-y2)^2]=14