Two similar circles have their radii in the ratio 5:8. What is the ratio of their areas?

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1. Area of a circle is $A=\pi r^2$.  
2. Let the radii be $5k$ and $8k$.  
3. Then $A_1=\pi(5k)^2=25\pi k^2$ and $A_2=\pi(8k)^2=64\pi k^2$.  
4. Cancel common terms to get $A_1:A_2=25:64$.  
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