Use sigma notation to express a left Riemann sum approximation (with 7 subintervals of equal width) of the integral below. Find the value of this approximation, rounding to the nearest 0.001:
a) Σ_{i=1}⁷ f(iΔx)
b) Σ_{i=0}⁶ f(iΔx)
c) Σ_{i=0}⁷ f(iΔx)
d) Σ_{i=1}⁶ f(iΔx)