March 8th, 2010, 08:09 AM  #1 
Member Joined: Jul 2009 Posts: 57 Thanks: 0  hard calc question
1) Suppose that f(t) is the amount of money in your bank account at time t. Explain in terms of spending and saving what would cause f(t) to be a) decreasing and concave down b) increasing and concave up c) decreasing and concave up 
March 8th, 2010, 08:36 AM  #2 
Senior Member Joined: Apr 2008 Posts: 435 Thanks: 0  Re: hard calc question
So what does a function look like when it's first derivative is positive and its second is negative? How about negative/negative? positive/positive? What about negative/positive? (this is the idea behind the question, no?) 
March 8th, 2010, 11:36 PM  #3 
Global Moderator Joined: Dec 2006 Posts: 20,653 Thanks: 2086 
Hint: the exponential function is increasing and concave up (a chord lies above the curve); I would hope the balance in your account is increasing exponentially, so what might that mean in terms of spending and saving?


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