Yellow
Yemen, yacht, yodel, yeast, yield, young, youth, years, yucca, yolks, yards, yawns, yikes...
No word in geometry state with the letter Y, but you need to ask your teacher if you can use a word that has a Y in it like; analytic geometry and complementary angles
To list a few: Yeomanly. Yielding. Young. Youthful. Yummy.