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LONDON—Archaeologists have discovered a smaller prehistoric(史前) site near Britain’s famous circle of standing stones at Stonehenge. Researchers have named the site “Bluehenge” after the color of the 27 Welsh stones that were laid to make up a path. The stones have disappeared, but the path of holes remains. The new circle, unearthed over the summer by researchers from Sheffield University, represents an important find, researchers said Saturday. The site is about a mile away from Stonehenge. Bluehenge, about 80 miles southwest of London, is believed to date back to about the （2010•连云港模拟）已知：如图，在等腰梯形ABCD中，AD∥BC，AB=DC，点E为边BC上一点，且AE=DC．（1）求证：四边形AECD是平行四边形；（2）当∠B=2∠DCA时，求证：四边形AECD是菱形．