什么是等比数列？

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一个故事了解等比数列~

const log2n = main(100, 2); // O(log2n) [ 1, 2, 4, 8, 16, 32, 64, 128 ]
sum(log2n, 2);
const log3n = main(100, 3); // O(log3n) [ 1, 3, 9, 27, 81, 243 ]
sum(log3n, 3);
const log4n = main(100, 4); // O(log4n) [ 1, 4, 16, 64, 256 ]
sum(log4n, 4);


function main(n, q = 2) {
  let i = 1;
  const returnVal = [i];
  while (i <= n) {
    i *= q;
    returnVal.push(i);
  }
  return returnVal;
}

//当q≠1时，前n项和公式为 [a1×(a1-an*q)]/(1-q)
function sum(geometricList, q = 2) {
  const a1 = geometricList[0], an = geometricList[geometricList.length - 1];
  const resultByFormula = a1 * (a1 - an * q) / (1 - q);
  console.log(eval(geometricList.join('+')));
  console.log('公式结果', resultByFormula);
  return resultByFormula;
}