.btn{
  display: inline-block;
  width:110px;
  line-height: 30px;
  text-align: center;
  background-color: $colorA;
  color:$colorG;
  border:none;
  cursor: pointer;
}
.btn-default{
  background-color: #B0B0B0;
  color:$colorG;
}
.btn-large{
  width:202px;
  height: 50px;
  line-height: 50px;
  font-size:18px;
}

为什么这样子写 btn-defaut就可以继承 .btn的样式
而不是像下面这样子写

.btn {
  display: inline-block;
  width: 110px;
  line-height: 30px;
  text-align: center;
  background-color: $colorG;
  color: $colorF;
  border: none;
  cursor: pointer;
}

.btn-default {
  @extend .btn;
  background-color: #b0b0b0;
  color: $colorG;
  border: 1px solid $colorF;
}