Two concentric circles are of radii 15 cm and 6 cm. Whatis the length (in cm) of the chord ofthe larger circle that is tangent to the smaller circle?

$6\sqrt{21}$

We have,

AO = 15 cm and OC = 6 cm

According to the Pythagoras theorem,

AC² = AO² - OC²

= AC² = 15² - 6² 

= AC² = 225 - 36

= AC² = 189

= AC = $3\sqrt{21}$

AB = 2AC = 2 x $3\sqrt{21}$ = $6\sqrt{21}$ cm