How would i find a piecewise linear approximation L(x) of f(x)=sin(2pi/2) defined on [0,4] with the partition being 0,1,2,3,4 ?

my book says to just join all data like {(x_0,f(x_0)), (x_1, f(x_1))...,(x_n,f(x_n))} but i'm confused with partitions, especially since this function doesn't give anywhere to plug in.

also there is another one: find the piecewise linear approx. L of f(x)=x^(1/2)+2 wrt the partition {0,1,4,9}