My grandpa has a farm. The farm is very big and beautiful. There are many animals on the farm. There are ten horses, seven goats, nine cows and many hens. The cows are black and white. The goats are white. The hens are very fat. My grandpa likes vegetables. So  there are a lot of potatoes, green beans, carrots and tomatoes. I like potatoes. My sister likes green beans. We all like this farm very much.

在水平地面上运动的小车车厢底部有一质量为$m_1$的木块，木块和车厢通过一根轻质弹簧相连接，弹簧的劲度系数为$k$.在车厢的顶部用一根细线悬挂一质量为$m_2$的小球．某段时间内发现细线与竖直方向的夹角为$\theta$，在这段时间内木块与车厢保持相对静止，如上图所示．不计木块与车厢底部的摩擦力，则在这段时间内弹簧的形变为 (　　)

____＋____ = ____