obtuse triangle

a triangle (polygon)

a right-angled triangle

acute triangle (triangle all of the angles of which are acute)

a tridecagon

(uranography) Winter Triangle - a bright asterism of the northern winter sky. Its corners are the stars Betelgeuse, Procyon, and Sirius.

(uranography) Summer Triangle - a bright asterism of the northern summer sky. Its corners are the stars Altair, Deneb, and Vega.

(geometry) area

(geometry) obtuse angle

to inform, to teach, to instruct / to tell