Browse · MATH
Printjmc
counting and probability senior
Problem
The sequence 2, 3, 5, 6, 7, 10, 11, contains all the positive integers from least to greatest that are neither squares nor cubes nor perfect fifth powers (in the form of , where is an integer). What is the term of the sequence?
Solution
We see that it's easier to count the number of perfect squares, perfect cubes and perfect fifth powers less than . We see there are 32 perfect squares less than 1089, which are , , , and then there are 10 perfect cubes which are , , . There are 4 perfect fifth powers less than 1089 which are , , . Then notice there are 3 numbers that are both perfect squares and perfect cubes which are 1, and . There are also 2 numbers that are both perfect squares and perfect fifth powers which are and . The only number which is both a perfect cube and a perfect fifth power is . The only number which is a perfect square, perfect cube, and perfect fifth power all at the same time is . So within the first 1089 positive integers we need to get rid of integers which means the term is .
Final answer
1041