以下各组岩石按成因属于同一类型的是（    ）

①花岗岩、砂岩  ②砾岩、页岩  ③砂岩、石灰岩  ④大理岩、页岩

科技小组的同学们设计了一个多档位的电热器模型，电路实物连接示意图如图所示，电路中电源两端电压保持不变，滑动变阻器的最大阻值为R₃．将滑动变阻器的滑片P置于中点M，且只闭合开关S₁时，电压表V₁的示数为U，电流表的示数I₁为0.8A；将滑动变阻器的滑片P置于B端，断开开关S₁，闭合开关S₂时，电压表V₁的示数为$\frac{3}{4}$U，电流表的示数为I₂，R₂消耗的电功率P₂为2.7W．

（1）求电流表的示数I₂；

（2）将滑动变阻器的滑片P置于A端，闭合开关S₁和开关S₂时，电压表V₂的示数为3U，求此时R₁消耗的电功率P₁．

   Arctic fox(北极狐) is a kind of small fox that lives in the Arctic. It grows about 50 centimeters long, not including its tail, 30 centimeters tall and weighs from 3 to 7 kilos.
   The Arctic foxes are white or blue in color. The white ones are white in winter, but grey-brown in summer. The blue ones are blue-grey all year round. The Arctic foxes have long, thick hair that is called fur. And the long fur covers their whole bodies, including both their cat eyes and rabbit feet, to keep them warm in the low temperature in the Arctic.
   The Arctic foxes feed mainly on birds, birds' eggs and other small animals. Teamwork is the way they look for food, and they don't like to do that alone.
   Baby Arctic foxes are very lovely. They don't open their eyes until a week after their birth. For the first half of the year, their parents offer them food. They begin to look for food on their own when they are about six months old. The next year they can live alone.
   Today, the number of the Arctic foxes is becoming smaller and smaller, because people kill them for their fur to make beautiful coats for money. Something must be done to protect the Arctic foxes.

已知$\cos \left(75°+\alpha \right)=\frac{1}{3}$，则cos（30°﹣2α）的值为____