〜ずとも
even without, even if (one) doesn't
Verb ない-stem + ずとも (する → せずとも)
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ずとも is a literary "even without doing": わざわざ言わずとも、分かっている, "you don't need to tell me, I know".
The form is the ない-stem + ず (a classical negative, as in ずに, N4) + とも ("even if"). So 言う → 言わずとも, 急ぐ → 急がずとも, and する → せずとも.
The second half usually says the result is fine anyway: 分かる, 間に合う, 治る, 大丈夫だ. It's the formal equivalent of なくても: 急がなくても間に合う = 急がずとも間に合う.
Don't confuse it with ずに, "without doing (and doing something else instead)": 朝ご飯を食べずに出かけた.
Examples
わざわざ言わずとも、分かっている。
You don't need to tell me. I know.
急がずとも、間に合うだろう。
We'll make it even if we don't hurry.
高い薬を使わずとも、治る病気だ。
It's an illness that gets better even without expensive medicine.
説明されずとも、見れば分かる。
You can tell just by looking, without it being explained.
そんなに心配せずとも、大丈夫だ。
It'll be fine, even if you don't worry so much.