Question 79849
Break the geometry down into a right triangle, take half of the 80cm chord length (40cm), and use it as one leg of the triangle. Use 30cm for the other leg, and solve for the hypotenuse (which will be the radius). 

you can use the Pythagorean Theorem to solve this: {{{a^2+b^2=c^2}}} and re-write to solve for "c"  {{{sqrt(a^2+b^2)=c}}}

Let a = 30
Let b = 40


{{{sqrt(30^2+40^2) = c}}}
{{{sqrt(900+1600) = c}}}
{{{sqrt(2500) = c}}}
{{{50 = c}}}
so the radius is 50 cm