As we know, if two circles intersect each other, then
∣r1−r2∣<C1C2<r1+r2 ........(i)
Now for the first circle C1(8,10) and r1=r
For the second circle C2(4,7) and r2=6
From (i)
∣r−6∣<5<r+6
⇒∣r−6∣<5...(ii) & 5<r+6...(iii) {|r-6|={\begin{matrix}r-6,r\geq 6 \\ 6-r,r<6\end{matrix}}
from (iii) r>−1...(iv)
from (ii)
when −1<r<6 then 6−r<5⇒1<r...(v)
when r≥6⇒r−6<5⇒r<11...(vi)
from (iv),(v),(vi) we get
⇒r∈(1,11)