fixieren
to fix
To attach something in a specific place or make it immobile.
Der Patient musste im Bett fixiert werden, um Stürze zu vermeiden.
The patient had to be fixed in the bed to prevent falls.

to fix / to stare at
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to fix
To attach something in a specific place or make it immobile.
Der Patient musste im Bett fixiert werden, um Stürze zu vermeiden.
The patient had to be fixed in the bed to prevent falls.

Verbs ending in '-ieren' often come from Latin or French. They are always regular and form the past participle without 'ge-' ('fixiert').
Wir haben den Termin für die Reise jetzt endgültig fixiert.
We have now definitively set the date for the trip.
You can recognize the meaning of 'fixieren' (to fix / to stare at) from the object: if it is about a physical object, it means 'befestigen' (to attach). If it is about eyes or the gaze, it means 'anstarren' (to stare at). If it is about an appointment or price, it means 'festlegen' (to set).
Der Mechaniker fixierte das Rad mit vier Schrauben.
The mechanic fixed the wheel with four screws.
In the meaning 'befestigen' (to attach), the position is often given with the preposition 'an' (+ dative). Typical objects are shelves, signs, bandages, or even patients.
Das Bild muss sicher an der Wand fixiert werden.
The picture must be fixed securely to the wall.
In the meaning 'festlegen' (to set), 'fixieren' is often used in office and business language. It sounds more binding and final.
Wir müssen die Agenda für das Meeting noch heute fixieren.
We still have to set the agenda for the meeting today.
This meaning is always used reflexively and with the fixed preposition 'auf' + accusative: 'sich auf jemanden/etwas fixieren' (to fixate on someone/something). It describes a strong, often unhealthy concentration.
Du solltest dich nicht nur auf deine Fehler fixieren.
You shouldn't fixate only on your mistakes.
The meanings connected with photography, chemistry, or biology are purely technical. Without context words like 'Foto' (photo), 'Labor' (laboratory), or 'Gewebe' (tissue), this meaning is unlikely.
Im Labor lernen die Studenten, wie man ein Präparat für das Mikroskop fixiert.
In the laboratory, the students learn how to fix a specimen for the microscope.
to solve / loosen
'lösen' and 'fixieren' are opposites when something is made fixed or free. 'fixieren' means to fasten something in one place or to set something in a binding way. 'lösen', on the other hand, means to loosen something, open it, or end a fixed connection. This can be physical, for example with a belt, or abstract, for example with a connection. In short: 'fixieren' makes something fixed, while 'lösen' removes that fixedness.

Wir fixieren den Termin nach dem Gespräch schriftlich.
We set the date in writing after the conversation.
Nach der Fahrt löst sie den Gurt und steigt aus.
After the ride, she unfastens the belt and gets out.