a right-angled triangle

a triangle (polygon)

a tridecagon

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

obtuse triangle

(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

please give me / (expressing a request) please do something for me

Please do not (do)