Mark M. answered 05/09/22
Retired math prof. Very extensive Precalculus tutoring experience.
Multiply f(each sample point) by the width of the interval in which the sample point lies, Then add up those products:
f(1.1)(1.3 - 1) + f(1.5)(1.7 - 1.3) + f(1.8)(2 - 1.7) = (1.1)(0.3) + (1.5)(0.4) + (1.8)(0.3)