`tend to do` 是正确的用法，意思是倾向于做某事。`tend` 是一个不及物动词，后面需要接动词不定式作为宾语。例如：
Sports cars tend to hold their value well.
After financial disasters, family members tend to do whatever they can to help each other and their communities.
而 `tend to doing` 是不正确的用法，因为 `tend` 后面不能直接跟动名词（即动词的-ing形式）。