User:Mathfantasia

My Contributions:

Divisibility rules: for 7

Rule 3: "Tail-End divisibility." Note. This only tells you if it is divisible and NOT the remainder. Take a number say 12345. Look at the last digit and add or subtract a multiple of 7 to make it zero. In this case we get 12370 or 12310. Lop off the ending 0's and repeat. 1237 ==> 123 ==> 130 ==> 13 NOPE. Works in general with numbers that are relatively prime to the base (and works GREAT in binary).

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Estimating Compound Interest

Look up and understand the "Rule of 72." My short synopsis here says that certain small percentages, say n%, will double in "72 divided by n" years.

I will now estimate between x=0 and x=1 by forming segments between the previous x values and x=.5. So for x<.5 we use 1 + .8x and otherwise 1.2x + .8.

I will now give an example of this method!

6% compounded yearly for 4 years with principal 20000. We see that 6% interest will make the principal double every 12 years (by the law of 72). But we only need 4 years, so with no more estimation we would need to know . We estimate this with the above linear equations. So we want to multiply the principal, 20000, by (). We end up getting 25336. And there ya go! Actual answer: 25250 (less than 1% off ... actually .3%)

Starting a K-12 Proofs and Comments page.