是省略了
补全了应该是：the stronger the value of the dollar【 is】,and the longer it stays strong,the weaker our economic activity 【will be】.
美元的价值越高,持续越久,我们的经济活动就会越低迷.
就像 ：the more ,the better
再举个例子：how many books do you want?the more the happier
其实是省略了：the more book I got ,the happier I will be