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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Remember the spoiler flag next time when posting solution-related stuff ~~
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I also can not commit my answer , but I hava a test in Chrome-Console,it pass,here is my code (javascript):
var circleArea = function(radius) {
if(typeof radius != 'number' || radius <=0) return false;
return (Math.PIradiusradius).toFixed(2);
};