mohon dibantu no 12 & 13

12)
ditanya : |a+2b| (cos 120 = -1/2);cosθ=cos120
jawab :
|a+2b|^2 = |a|^2 + |2b|^2 + 2|a||2b|cosθ
= 8^2 + 12^2 + 2.8.12.-1/2
=64 + 144 -96
|a+2b|^2= 112
|a+2b| = akar112
= 4akar7 [B]

13)
ditanya : |a-b|^2 (cosθ=cos60=1/2)
jawab :
|a-b|^2= |a|^2 + |b|^2 - 2|a||b|cosθ
= 4^2 + 3^2 - 2.4.3.1/2
= 16 + 9 - 12
= 13 [B]