counting factors of N

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Let N= 2^2 \times 3^3 \times 7^4. How many factors of N^2 which are less than N,are not factors of N


The answer is 98.

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2 solutions

Piyushkumar Palan
Dec 29, 2013

Given that

N = 2 2 × 3 3 × 7 4 N = 2^2 \times 3^3 \times 7^4

N 2 = 2 4 × 3 6 × 7 8 \Rightarrow N^2 = 2^4 \times 3^6 \times 7^8

Number of positive factors of N = ( 2 + 1 ) ( 3 + 1 ) ( 4 + 1 ) = 60 N = (2+1)(3+1)(4+1) = 60

Number of positive factors of N 2 = ( 4 + 1 ) ( 6 + 1 ) ( 8 + 1 ) = 315 N^2 = (4+1)(6+1)(8+1) = 315 which includes N N and half each of remaining as < N < N and > N > N .

Number of positive factors of N 2 N^2 which are N = ( 315 + 1 ) / 2 = 158 \leq N = (315 + 1) / 2 = 158

Number of positive factors of N 2 N^2 which are less than N N ,are not factors of N = 158 60 = 98 N = 158 - 60 = \boxed {98}

I don't understand!!! Sorry

Aparna Kalbande - 5 years, 6 months ago

TOTAL FACTORS OF N^2=315 . FOR EVERY VALYE BETWEEN 1 AND N THERE EXIST ANOTHER VALUE BETWEEN N AND N^2 SUCH THAT THERE GEOMETRIC MEAN IS N. THUS IF WE REMOVE 1,N,N^2 FROM THE 315 WE WILL HAVE 1............N............N^2 ,,,,,,,,, 156 TERMS BETWEEN 1 AND N ,AND, N AND N^2 TOTAL FACTORS OF N=60 . REMOVING 1 AND N FROM FACTORS WE GET FACTORS OF N=58 .NOW THE ANSWER =156-58=98

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