[기초 수학 지식] if vs only if
좋은 설명 되있음 https://www.khanacademy.org/test-prep/lsat/lsat-lessons/logic-toolbox-new/a/logic-toolbox--if-and-only-if The Logic of "If" vs. "Only if" (article) | Khan Academy Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for a..