12. Let w(x) be the number of prime numbers less than or equal to x. Compute fo r(x)dx. 13. Is the statement below true, or untrue? If true, explain why. If false, give a counterexample. If f, g : [a, b] - R are both not integrable, then f + g is not integrable. 14. The figure below shows the graph of an integrable function f : [0, c] - R. It is given that f(x) dx = 2 and If(x) |dx = 4. Determine f(z)dx. Carefully explain your reasoning. h