The angle between vectors a = 2i + j and b = i + 2j is
A30°
B45°
C53.13°
D36.87°CORRECT
GCEFurther MathematicsVectors2019
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Why the answer is D, and why the others tempt you.
cos θ = (a·b)/(|a||b|) = (2×1 + 1×2)/(√5 × √5) = 4/5 = 0.8. Therefore θ = cos⁻¹(0.8) ≈ 36.87°.
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