WAEC Further Mathematics 2010 Paper

$a \cdot b = |a||b| \cos \theta$

$a = 3i - 4j;\ b = 6i + 4j$

$18 - 16 = \left(\sqrt{3^{2} + (-4)^{2}}\right)\left(\sqrt{6^{2} + 4^{2}}\right) \cos \theta$

$2 = 5\sqrt{52} \cos \theta$

$\cos \theta = \frac{2}{5\sqrt{52}} = 0.0555$

$\theta = 86.8^{\circ} \approx 87^{\circ}$