Final answer:
The quotation "Et tu, Brute?" from Shakespeare's play is not reliable for historical analysis on Caesar's death; historians use primary sources like writings of ancient Roman historians Suetonius or Plutarch.
Step-by-step explanation:
The quotation "Et tu, Brute?" is from William Shakespeare's play Julius Caesar and not a primary historical source. While it captures the dramatic betrayal of Caesar by his friend Brutus, it was written over 1,500 years after the actual event. Therefore, this quotation is not a reliable source for historians writing about the death of Julius Caesar. Historians prefer to reference contemporary accounts and primary sources, such as the writings of ancient Roman historians like Suetonius or Plutarch, both of whom chronicled the life of Caesar and the events surrounding his assassination.