〜たところで
even if (it'd be pointless)
Verb た-form + ところで
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たところで means "even if you did", with the strong implication that it wouldn't help: 今から急いだところで、間に合わない, "even if we hurry now, we won't make it".
The second half is negative or pessimistic: it won't change, won't help, is too late, is only a small amount. It's often paired with いくら or どんなに: いくら説明したところで、彼にはわからない.
It always follows a た-form, even for the future. That's what distinguishes it from たところ (N3), "when I did (and found)", which is about something that actually happened.
It can also downplay an amount: 失敗したところで、大した損はない, "even if it fails, it's no big loss".
Examples
今から急いだところで、間に合わない。
Even if we hurry now, we won't make it.
謝ったところで、許してもらえないだろう。
Even if I apologise, they probably won't forgive me.
いくら説明したところで、彼にはわからない。
No matter how much I explain, he won't understand.
心配したところで、何も変わらない。
Worrying won't change anything.
今さら後悔したところで、もう遅い。
It's too late for regrets now.