A Minus Times a Minus

Posted in math at 7:32 pm by danvk

Back in grade school, I never got a satisfactory answer to the question “why is a minus times a minus equal to a plus?”

I didn’t think about it again for years, until my mom asked for an explanation to give when she taught her students. This time around, I immediately realized how you’d go about proving it. Here’s how it goes:

1 + (-1) = 0   (definition of “-1″)
(-1) * 1 + (-1) * (-1) = 0   (multiply both sides by -1 and distribute)
-1 + (-1)(-1) = 0   (anything times 1 is itself)
(-1)(-1) = 1   (add 1 to both sides)

Easy as could be, right? Unfortunately, mom preferred my brother’s explanation:

A minus times a minus is equal to a plus.
The reasons for this, we need not discuss.

FreeCreditReport.com scam

Posted in personal, web at 7:16 pm by danvk

I was looking over my checking account transactions for the past year and saw a mysterious $12.95 charge from “CIC*Triple Advantage 877-48168″ appearing once a month. A search quickly showed that I’d been had by FreeCreditReport.com.

Turns out I’d used this site to get a credit check when I moved in to my old apartment last August. Everyone is entitled to one free credit check per year, and I thought that this was what I was getting. I’d failed to notice the fine print on their main page (helpfully displayed in blue-on-blue text):

When you order your free report here, you will begin your free trial membership in Triple AdvantageSM Credit Monitoring. If you don’t cancel your membership within the 7-day trial period†, you will be billed $14.95 for each month that you continue your membership.
ConsumerInfo.com, Inc. and Freecreditreport.com are not affiliated with the annual free credit report program. Under a new Federal law, you have the right to receive a free copy of your credit report once every 12 months from each of the three nationwide consumer reporting companies. To request your free annual report under that law, you must go to www.annualcreditreport.com.

That’s right, they sign you up for a free trial of a monthly service which you must opt-out of after 30 days. They do their best not to tell you that they’ve signed you up.

Looking back in my checking account, I found 16 months * $12.95/month = $207.20 in charges. Thanks to help from this discussion, I was eventually able to get this refunded in its entirety. Read more for details.

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Now and Then

Posted in san francisco at 11:06 pm by danvk

I plugged in my cross streets on the San Francisco Library’s digital photo archive and found this picture. The image is mislabeled. The street in the foreground is actually Fillmore, not Webster. Which means that this photo, from 1945, was taken from my roof!

This past weekend, Miriam and I did a Now and Then shot. There’s a higher-resolution image there, but you can fade between them using the slider here:


There are more trees now, more cars and more power lines, but one less cable car. But the coolest difference is only noticeable if you look at the tall building in the upper right. It hasn't shrunk. No, the trees in Buena Vista Park have grown!