What are some situations in which estimating a sum or difference is useful? why is it useful in these situations?

The surface area (sa) of a cube with a as the length of each of its sides is given by the formula . if the surface area is known, how can you rewrite the formula to find its side?

Suppose there is a simple index of three stocks, stock abc, stock xyz, and stock qrs. stock abc opens on day 1 with 4000 shares at $3.15 per share. stock xyz opens on day 1 with 5000 shares at $4.30 per share. stock qrs opens on day 1 with 6000 shares at $4.60 per share. the price of stock abc on day 8 begins at $3.50. the price of stock xyz on day 8 begins at $3.90. stock qrs opens on day 8 with a price of $4.50 per share. assume that each stock has the same number of shares that it opened with on day 1. what is the rate of change of this simple index over 1 week?

You deposit $5000 each year into an account earning 3% interest compounded annually. how much will you have in the account in 30 years?