Wave Optics

In Young's double slit experiment, the intensities at two points, for the path difference $\frac{\lambda}{4}$ and $\frac{\lambda}{3}$ are $\mathrm{I}_1$ and $\mathrm{I}_2$ respectively. If $\mathrm{I}_0$ denotes the intensity produced by each one of the individual slits then the ratio $\left(\mathrm{I}_1+\mathrm{I}_2\right): \mathrm{I}_0$ is $\left(\cos 45^{\circ}=1 / \sqrt{2}\right)\left(\cos 60^{\circ}=0.5\right)$