if {u1, u2, u3, u4} spans r3, then so does {u1, u2, u3}. I know the answer is true, but I want to know why. I understand that {u1,u2,u3} span R3, but what makes u4 dependent to u1,u2,u3? why I cannot pick a random u4 which satisfied u4 ≠ au1+bu2+cu3,
