Potato chips are Americans' favorite snacks(零食). Where do potato chips come from? In 1853 a native(本土) American named George Crum made the first potato chips. Crum was a chide(头目) in an expensive restaurant in Saratogasprings， New York. One day， a customer(顾客) did not like the French fries. He said they were too thick(厚的). So Crum made more， this time thinner. The customer still didn't like them. Crum got unhappy. He decided to make the customer angry. So he cut the potatoes， very thin. The customer loved them.

    Other people wanted Crum's  potato chips. Now， there was a new food on the menu— Saratoga chips. Soon Crum opened his own restaurant and made his special chips.

    It took a long time to make potato chips. People peeled the potatoes by hand. Then in 1920，the automatic(自动的) potato peeler changed everything. Now it was faster and easier to make potato chips. Now potato chips were not a specialty. They were a popular snack food， but only in the north.

    A salesman named Herman Lay wanted to sell potato chips in the south. He sold potato chips in bags. His business grew. Today， Americans eat a lot of potato chips every day.

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①a＞b，c＜d⇒a﹣c＞b﹣d；

②a＞b＞0，c＜d＜0⇒ac＞bd；

③a＞b＞0⇒ $\sqrt[3]{a}>\sqrt[3]{b}$ ；

④a＞b＞0⇒ $\frac{1}{a^2}>\frac{1}{b^2}$ ．