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What is the number of perfect 3 0 th 30^{\text{th}} powers that divides 1 ! × 2 ! × 3 ! × × 30 ! 1! \times 2! \times 3! \times \ldots \times 30! ?


The answer is 588.

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1 solution

1 ! 2 ! 3 ! 4 ! . . . 29 ! 30 ! = ( 1 ) ( 1.2 ) ( 1.2.3 ) ( 1.2.3.4 ) . . . . ( 1.2.3...29 ) ( 1.2.3....29.30 ) 1!2!3!4! ... 29!30! = (1)(1.2)(1.2.3)(1.2.3.4) .... (1.2.3 ...29)(1.2.3. ... 29.30)

= 1 30 . 2 29 . 3 28 . 4 27 . . . 2 9 2 . 3 0 1 = 1^{30}.2^{29}.3^{28}.4^{27} ... 29^2.30^1

= 2 ( 29 + 27 + 25 + 23 + . . . + 1 ) + ( 27 + 23 + 19 + 15 + . . . + 3 ) + ( 23 + 15 + 7 ) + ( 15 ) = 2^{(29+27+25+23+...+1)+(27+23+19+15+...+3)+(23+15+7)+(15)}

× 3 ( 28 + 25 + 22 + . . . + 1 ) + ( 22 + 13 + 4 ) + ( 4 ) × 5 ( 26 + 21 + 16 + . . . + 1 ) + ( 6 ) \times 3^{(28+25+22+...+1)+(22+13+4)+(4)} \times 5^{(26+21+16+...+1)+(6)}

× 7 ( 24 + 17 + 10 + 3 ) × 1 1 ( 20 + 9 ) × 1 3 ( 18 + 5 ) × 1 7 14 × 1 9 12 × 2 3 8 × 2 9 2 \times 7^{(24+17+10+3)} \times 11^{(20+9)} \times 13^{(18+5)} \times 17^{14} \times 19^{12} \times 23^8 \times 29^2

= 2 225 + 105 + 45 + 15 × 3 145 + 39 + 4 × 5 81 + 6 × 7 54 × . . . . . = 2^{225+105+45+15} \times 3^{145+39+4} \times 5^{81+6} \times 7^{54} \times ..... can be ignored as powers are < 30 <30

= 2 390 3 188 5 87 7 54 . . . = 2^{390}3^{188}5^{87}7^{54} ... = ( 2 30 ) 13 . ( 3 30 ) 6 . ( 5 30 ) 2 . ( 7 30 ) 1 . . . = (2^{30})^{13}.(3^{30})^6.(5^{30})^2.(7^{30})^1... primes with powers < 30 <30

So the number is perfect 3 0 t h 30^{th} power of each term in expansion of

( 2 0 + 2 1 + 2 2 + . . . 2 13 ) ( 3 0 + 3 1 + 3 2 + . . . 3 6 ) ( 5 0 + 5 1 + 5 2 ) ( 7 0 + 7 1 ) (2^0+2^1+2^2+...2^{13})(3^0+3^1+3^2+...3^6)(5^0+5^1+5^2)(7^0+7^1)

14.7.3.2 = 588 \implies 14.7.3.2 = \boxed{588} perfect 3 0 t h 30^{th} power divisors

owesome!!!!!! sir

shyam kawale - 4 years, 10 months ago

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