The early Silk Roads facilitated trade and the spread of ideas, technology, disease and religion across Eurasia in the beginning of the Common Era. Practice this yourself on Khan Academy right now: https://www.khanacademy.org/humanities/world-history/ancient-medieval/silk-road/e/key-concepts--silk-road World History on Khan Academy: From the earliest civilizations to the modern world, geography, religion, trade, and politics have bound peoples and nations together — and torn them apart. Take a journey through time and space and discover the fascinating history behind the complex world we inhabit today. About Khan Academy: Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content. For free. For everyone. Forever. #YouCanLearnAnything ... https://www.youtube.com/watch?v=lLeIclx2lAU Note: All Khan Academy content is available for free at (www.khanacademy.org)

The position of a particle moving in the xy-plane is given by the position vector (-3t_+4t_,t_+2). Sal analyzes it to find the acceleration vector of the particle at time t=3. Watch the next lesson: https://www.khanacademy.org/math/ap-calculus-bc/bc-applications-derivatives/bc-planar-motion-diff/v/motion-along-a-curve-example-1?utm_source=YT&utm_medium=Desc&utm_campaign=APCalculusBC Missed the previous lesson? https://www.khanacademy.org/math/ap-calculus-bc/bc-applications-derivatives/bc-optimization/v/extreme-derivative-word-problem-advanced?utm_source=YT&utm_medium=Desc&utm_campaign=APCalculusBC AP Calculus BC on Khan Academy: Learn AP Calculus BC - everything from AP Calculus AB plus a few extra goodies, such as Taylor series, to prepare you for the AP Test About Khan Academy: Khan Academy is a nonprofit with a mission to provide a free, world-class education for anyone, anywhere. We believe learners of all ages should have unlimited access to free educational content they can master at their own pace. We use intelligent software, deep data analytics and intuitive user interfaces to help students and teachers around the world. Our resources cover preschool through early college education, including math, biology, chemistry, physics, economics, finance, history, grammar and more. We offer free personalized SAT test prep in partnership with the test developer, the College Board. Khan Academy has been translated into dozens of languages, and 100 million people use our platform worldwide every year. For more information, visit www.khanacademy.org, join us on Facebook or follow us on Twitter at @khanacademy. And remember, you can learn anything. ... https://www.youtube.com/watch?v=HFdeHX_wA4k Note: All Khan Academy content is available for free at (www.khanacademy.org)