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如图,为了测量山坡上一棵树PQ的高度,小明在点A处利用测角仪测得树顶P的仰角为450,然后他沿着正对树PQ的方向前进100m到达B点处,此时测得树顶P和树底Q的仰角分别是600和300,设PQ垂直于AB,且垂足为C.

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    Alaska's state fair, which runs until September 5th, began as a celebration among residents of the Mantanuska Colony, a project under which 200 farm families were moved to Alaska to see whether agriculture could be possible in the coldest state. The state fair lives on, but little more than a decade after the start of the project most of the participants had abandoned their farms. The project was widely seen as a flop.

    In this state, glaciers cover 300 times more acres than farms. Only 5% of the food consumed is grown locally, compared with 81% nationwide. The growing season is short and summer temperatures chilly. Tomato plants wither(凋谢). Fruit trees, in most parts of the state, are just a dream.

    Enter the high tunnel: a greenhouse consisting of a metal frame with plastic stretched across it. Its few millimeters of plastic separate crops inside from the great outdoors. But this is enough for Alaskan growers to produce tomatoes as well as sweetcorn, peaches and kiwi fruit, and to boost production of crops by a quarter or more.

    In a place where no one blinks(眨眼) if you call yourself a fisherman, boat captain or gold miner, an increasing number of Alaskans are thinking of themselves as people who grow food. Since the start of the programme, the number of farms registered(登记)with the state has nearly doubled. Local restaurants have begun shaping their menus around what neighboring farms can grow.

    Eight decades ago, the Mantanuska Colony tried to turn farmers into Alaskans. Today, the high tunnels are turning Alaskans into farmers.