Euclid:

Further, of trilateral figures, a right-angled triangle is that which has a right angle, an obtuse-angled triangle that which has an obtuse angle, and an acute-angled triangle that which has its three angles acute.

— Euclid Ancient

Father of geometry

Context

Definition in 'Elements'

Date / Period

300 BC

Source

https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI21.html

Verification

Confirmed

Explanation

Found in 2 providers: deepseek,grok

Method

Cross Reference

Sources Checked

2 sources

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