guys please help me answer my homework

Mapel : Matematika
Kelas : X SMA
Bab : Vektor

Pembahasan :
2.

ab = |a| |b| Cos 60°
[2 1 -3] [-1 x -2] = √14 √(x² + 5) (1/2)
-2 + x + 6 = √(14x² + 70) (1/2)
2(x + 4) = √(14x² + 70)
2x + 8 = √(14x² + 70)
(2x + 8)² = 14x² + 70
4x² + 32x + 64 = 14x² + 70
10x² - 32x + 6 = 0
5x² - 16x + 3 = 0
(5x - 1)(x - 3) = 0
x = 1/5
x = 3 —» Bulat

Jawaban : D

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3.

ab = |a| |b| Cos 60°
[x 1 -3] [-1 3 -2] = √(x² + 10) √14 (1/2)
-x + 3 + 6 = √(14x² + 140) (1/2)
2(-x + 9) = √(14x² + 140)
-2x + 18 = √(14x² + 140)
(-2x + 18)² = 14x² + 140
4x² - 72x + 324 = 14x² + 140
10x² + 72x - 184 = 0
5x² + 36x - 92 = 0
(5x + 46)(x - 2) = 0
x = -46/5
x = 2

Jawaban : E