题目3.证明:如果向量a.b共线,那么向量 2a+b 与a共线.-|||-4.如图,已知四面体ABCD的所有棱长都等于a,E,F,G分别是-|||-棱AB,AD,DC的中点.求:-|||-(1)AB·AC;-|||-(2)→(AD)·→(DB);-|||-(3)→(GF)·→(AC);-|||-(4) →(EF)·→(BC)-|||-(5)→GG·→BA;-|||-(6)→(GE)·→(GF)-|||-F-|||-B D-|||-G-|||-C题目解答答案