向量是既有大小又有方向的量,向量的加法有两种法则,三角形法则和平行四边形法则.三角形法则主要体现出首尾相连形成一个三角形,方向是第一个向量的起点指向第二个向量的终点。平行四边形法则是形成一个平行四边形,其对角线为和,且三个向量是同一个起点.向量加法的两个法则能够灵活应用是解决向量问题的关键.例:如图四边形ABCD,E,F分别是AD,BC的中点,求证: Vector is both the size and direction of the vector, there are two kinds of vector addition law, the law of triangles and parallelogram law. Triangle law mainly reflects the formation of a triangle connected end to end, the direction is the starting point of the first vector points to the second vector The end Parallelogram method is to form a parallelogram, the diagonal and, and three vectors are the same starting point. Vector addition of the two laws can be flexible application of the key to solve the vector problem. Example: quadrilateral ABCD, E, F are AD, the midpoint of BC, verify: