Question 1117484
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First make a sketch . . . 


You will see a right angled triangle with the legs "one half the radius", "one half of the chord" and the hypotenuse = "radius".


Let x = half of the radius.

Then from Pithagorean theorem you have


{{{x^2 + 15^2}}} = {{{(2x)^2}}}

{{{x^2 + 225}}} = {{{4x^2}}

225 = {{{3x^2}}}  ====>  {{{x^2}}} = {{{225/3}}} = 75  ====>  x = {{{sqrt(75)}}} = {{{5*sqrt(3)}}}.


The length of the radius = 2x = {{{10*sqrt(3)}}}.
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Solved.