AB is the tangent to both the circles. ⇒ AB⊥AR Let SC be the line parallel to AB. ⇒ SC⊥AR and AC=BS=5cm and AB=SC Radius of the bigger circle, AR=20cm ⇒ AC+CR=20⇒5+CR=20 ⇒ CR=20−5=15cm and RS=25cm lnΔRCS, SC2=RS2−CR2=252−152 SC=√625−225 =√400=20cm ∴ AB=SC=20cm