Sb / Sth be said to do ...这个句型解释为“据说……；人们认为……”,是由It is said that sb/sth do...转化而来的.除了said这个动词以为,在这个句型中还可以使用reported / belived / supposed等动词,意思都可以解释为“据报导……；人们相信……；大家认为……”.
例如：It is reported that the storm damaged the city seriously.
= The storm is reported to have damaged the city seriously.
It is believed that our city football team will win the final.
= Our city football team is believed to win the final.
It is said that the writer is writing a long novel.
= The writer is said to be writing a long novel.
It is believed that the carved stone head was put upside down deliberately.
= The carved stone head is believed to have been put upside down deliberately.
你说的那个句子的主语是An economy,而且还带了一个whose 引导的定语从句（whose ...expanding),然后再跟上is said to have succumbed ...
整句话的意思是：当某个经济体的这种产出能力不再扩大,人们就认为它遭遇了经济停滞.
