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Primes less than 20 2, 3, 5, 7, 11, 13, 17, 19
Primes greater than 20 23, 29, 31, 37, 41, 43, 47
Divisibility by 3 sum of digits divisible by 3
divisibility by 4 last 2 digits divisible by 4
divisibility by 7 difference b/t units digit x 2 and rest of number divisible by 7
divisibility by 8 last 3 digits divisible by 8
divisibility by 9 sum of digits divisible by 9
divisibility by 11 difference between odd digits places summed and even digits places summed divisible by 11
Simple interest P(1+rt)
compound interest P(1+r)^t
interest: p or principal starting investment amount
interest: r interest rate per time period
interest: t number of time periods interest is accrued over
14^2 196
16^2 256
17^2 289
18^2 324
19^2 361
a^2 - b^2 (a+b)(a-b)
a^2 + 2ab + b^2 (a+b)^2
a^2 - 2ab + b^2 (a-b)^2
odd + odd even
even + even even
odd + even odd
odd x odd odd
even x even even
odd x even even
odd ^ positive odd
even ^ positive even
first three pythagorean triples 3-4-5, 5-12-13, 8-15-17
lengths of isosceles right triangle 1-1-sqrt2
lengths of 30-60-90 triangle 1-2-sqrt3
sum of interior angles of a polygon 180(n-2)
diagonal of rectangular prism l^2 + w^2 + h^2
volume of cylinder pi r^2 h
surface area of cylinder 2 pi r^2 + 2 pi r h
volume of sphere 4/3 pi r^3
surface area of sphere 4 pi r ^2
decimal 1/6 0.1666
decimal 1/8 0.125
decimal 1/9 0.1111...
decimal 5/6 0.8333...
decimal 3/8 0.375
decimal 5/8 0.625
decimal 7/8 0.875
decimal 1/11 0.090909...
total time to finish a job (2 people) (TaTb)/(Ta + Tb)
total time to finish a job (2+ people) 1/Tall = 1/Ta + 1/Tb + 1/Tc + ...
does order matter for combo or permu? order matters for permutations
formula for combinations: n choose k (how many combos of k items from n set) n! / (k! (n-k)!)
formula for permutations n! / (n-k)!
formula for arrangements of repeating elements where rx repeats n! / r1! r2! ...
Created by: qiguai