AB=15 BC=25 AC=x, then We know that the sum of two sides of a triangle is always greater than third side. ⇒ AB+BC>x ⇒ 15+25>x ⇒ 40>x ...(i) Also, the differences of two sides is always less than third side. BC−AB<AC 25−15<x 10<x ...(ii) From eq. (i) and (ii) 10<x<40