1To throw or cast — e.g., to cast (a vote), to cast light or a shadow.
彼は大統領選で支持候補に一票を投じた。
He cast his vote for his preferred candidate in the presidential election.
橋のライトが川面に光を投じていた。
The bridge lights were casting light onto the surface of the river.
その発言は会議に多くの疑問を投じた。
The remark cast many doubts at the meeting.
2To devote oneself to something; to throw oneself into (often used as 身を投じる).
彼は途上国支援に身を投じた。
He devoted himself to supporting developing countries.
若い研究者は新しい分野の研究に身を投じ始めた。
The young researcher began to devote himself to studying a new field.
3To invest or put in (money, time, effort) — to allocate resources to a project or endeavor.
会社は新規事業に多額の資金を投じた。
The company invested a large sum of money in the new business.
彼女はプロジェクト成功のために時間を投じた。
She devoted time to ensure the project's success.
4To administer (medicine) or to apply treatment; used in medical contexts.
医師は患者に新薬を投じた。
The doctor administered a new drug to the patient.
治療チームは集中的な治療を投じて回復を図った。
The treatment team applied intensive therapy in an attempt to bring about recovery.
5To abandon or surrender; used in literary/idiomatic contexts to indicate giving up resistance.
守備隊はついに抵抗を投じ、城は降伏した。
The garrison finally gave up resistance and the castle surrendered.
6To make full use of or take advantage of (resources, experience, etc.).
彼は長年の経験を投じてプロジェクトを成功に導いた。
He used his years of experience to lead the project to success.
7To fit well with or be in tune with something; used in contexts meaning to match or harmonize.
新しいデザインは会社のブランドイメージにうまく投じている。
The new design fits well with the company's brand image.
8To stay at a lodging (archaic or literary usage).
旅人たちは一晩その宿に投じて休んだ。
The travelers stayed at the inn for one night and rested.