Solution: The angle $\theta$ between $\mathbfa$ and $\mathbfb$ is given by $\cos\theta = \frac\mathbfa \cdot \mathbfb\$. Compute the dot product: $2(1) + (-3)(4) = 2 - 12 = -10$. Compute magnitudes: $\|\mathbfa\| = \sqrt2^2 + (-3)^2 = \sqrt13$, $\|\mathbfb\| = \sqrt1^2 + 4^2 = \sqrt17$. Thus, $\cos\theta = \frac-10\sqrt13\sqrt17$. Rationalizing, $\theta = \arccos\left(-\frac10\sqrt221\right)$. $\boxed\arccos\left(-\dfrac10\sqrt221\right)$ - Portal da Acústica