写一个函数， 求1到100的和

// 等差数列求和法
function sumArea (first, last) {
  return (first + last) * (last - first + 1) / 2
}
sumArea(1,100) // 5050
sumArea(200,300) // 25250

// 循环求和法
function sumArea(first, last) {
   var sum = 0
   for (var i = first; i <= last; i++) {
      sum += i
   }
   return sum
}
sumArea(1,100) // 5050
sumArea(200,300) // 20250