quicky on a math part of GRE (review)

[sel164]

Hey Guys,

I found something that can potentially be useful to those who'll be taking the GRE.

How to recognize Multiples of 2, 3, 4, 5, 6, 9, and 10.
2: Last digit is even.
3: Sum if digits is multiple of 3.
4: Last two digits are multiple of 4.
5. Last digit is 5 or 0.
6. Sum of digits is multiple of 3 and last digit is even.
9: Sum of digits is multiple of 9.
10: Last digit is 0.

How to find a common multiple.
The product is the easiest common multiple to find. If the two numbers have any factors in common, you can divide them out of the product to get lower common multiple.
Ex. What is the least common multiple of 28 and 42?
28 x 42=1,176
28=2 x 2 x 7 and 42=2 x 3 x 7
They both share a 2 and a 7, so divide the product by 2 and then by 7. 1,176 / 2=558. 558 / 7=84. So, the least common multiple is 84.

How to find the average of consecutive numbers or evenly spaced numbers.
The average if evenly spaced numbers is simply the average of the smallest number and the largest number.
The average of all odd integers from 11 to 77, is the same as the average of 13 and 70, and is the same as the average of all numbers from 13 to 77 inclusive.
(13+77) / 2 = 45.

How to find one angle or the sum of all the angles or a regular polygon (don't forget that in polygon all sides are equal!).
Sum of all the interior angles in a polygon with n sides = (n-2) x 180.
Degree of measure of one angle in a Regular Polygon with n sides = [(n-2) x 180] / n.

How to find the LENGTH of and ARC.
LENGHT of an arc = (n / 360) x 2 (Pi) (r)

How to find the VOLUME of a SPHERE.
Volume= (4 / 3) (Pi) (r^3)