tolong bgt dibntu gimna carangrjainnya

4) |CD| = 2|BD| => CD/BD = 2/1, G titik tengah AC
GD = GC + CD
GD = 1/2 AC + 2/3 CB
GD = 1/2 AC + 2/3 (CA + AB)
GD = 1/2 AC + 2/3 (-AC + AB)
GD = 1/2 AC - 2/3 AC + 2/3 AB
GD = 2/3 AB - 1/6 AC
GD = r AB + s AC ==> r = 2/3, s = -1/6
r + s = 2/3 + (-1/6) = 3/6 = 1/2

5) gambar segienam terlebih dahulu dengan pusat O
AB = u => AB = FO = OC = ED = u
AF = v => AF = BO = OE = CD
AO = u + v => AO = OD = FE = BC = u + v
AB + AC + AD + AE + AF
= AB + (AB + BC) + (AO + OD) + (AF + FE) + AF
= u + (u + (u + v)) + ((u + v) + (u + v)) + (v + (u + v)) + v
= 6u + 6v

6) a - b = (1, 3, 3) - (3, 2, 1) = (-2, 1, 2)
a + c = (1, 3, 3) + (1, -5, 0) = (2, -2, 3)
(a - b)(a + c) = -2(2) + 1(-2) + 2(3) = -4 - 2 + 6 = 0
Karena hasil kali ya 0 maka (a - b) tegak lurus (a + c) sehingga sudut antara (a - b) dengan (a + c) = 90°