This is question that was tested in ACJC H2 Math Prelim P1 2015. A few of my students know how to answer it but were uncertain how to express it.
Most students were concerned with (ii) of the question.
I think the easiest way to prove this, is to first avoid writing too much and attempt to show it mathematically.
Thus will be smaller in magnitude.
Students can also attempt to justify using the area under graph but they must express the answers in words carefully.