Amplitudes of Intensity for Single slit and double slit diffraction?

Question

Why are the amplitudes of intensity of the pattern generated by single slit diffraction different (gradually decrease as you go away from the center) and yet with double-slit diffraction the intensity of maxima are constant regardless of distance from the center?

Answer 1

You get constructive interference with two or more slits but not with one. (See ref. 1.) Looking at interference only, you see constant peaks. However when you combine diffraction with interference you get a central group of maxima that is larger than the groups farther from the center. Ref. 2 lets you switch between numbers of slits and total or interference-only amplitudes.