Hello,
What is the difference between 関係(かんけい)and 関連(かんれん)?
It must be difficult to answer, even in Japanese!
First of all, please have a look at both Kanji carefully.
The second Kanjis are「係」and 「連」.
In Japanese 「かかり:かかわる:」and「れん:つらなる」
So, it supposed be 関係 means かかわる+かかわる、関連 means かかわる+つらなる。
関係 has wider range of meaning than 関連.
関係 means some relation between more than two things, or influenced from the other .
On the other hand, 関連 also express some connection between more than two matters, but in this case it has to be connected as contents of the matter.
Ex: 警察(けいさつ)は先週起きたふたつの事件の関連/関係を調べている。
(The police is investigating the connection/relation between two cases which happened last week.)
Both 関係 and 関連 can be used.
Ex: 彼はこの事件に関係しているようだ。(関連×)
He seems that he is related to this case.
I hope it will help you.
がんばってくださいね。