在四边形ABCD中，若$\overrightarrow{AB}+\overrightarrow{AD}=\overrightarrow{AC}$，且$\left(\overrightarrow{AB}+\overrightarrow{AD}\right)·\left(\overrightarrow{CB}-\overrightarrow{CD}\right)=0$，则（ ）

如图所示，一质量M=40kg、长L=2.5m的平板车静止在光滑的水平地面上．一质量m=10kg可视为质点的滑块，以v₀=5m/s的初速度从左端滑上平板车，滑块与平板车间的动摩擦因数μ=0.4，取g=10m/s²．

如果关于x的多项式 x⁴+（a-1）x³+5x²-（b+3）x-1不含x³项和x项，求a、b的值．

—It was fun; you ________.

    July is coming. People all around the world are talking about their____(vacation). Parents would like____(help) their children relax.____ (luck), they have two months off. Some people enjoy quiet life, so many of ____(they) stay at home watching TV, using computers and so on.____some spend time differently. My pen pal Tony ____ blonde hair is a boy from the US. He says that ____(America) like going to the countryside. Just like his big family, they often go to a farm. There, some can feed animals, some can play and ____(other) can watch____ nice sky and rivers. They can also taste delicious food. They are quite _____(interest) in the life there.