Now generalize this

Algebra Level 5

2 × 3 × 4 × × 99 \large\left\lceil\sqrt2\right\rceil\times\left\lceil\sqrt3\right\rceil\times\left\lceil\sqrt4\right\rceil\times\cdots\times\left\lceil\sqrt{99}\right\rceil

Find the largest non-negative integer n n such that 2 n 2^n is a factor of the number above.


Inspiration .


The answer is 91.

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3 solutions

Let the product be P P , then we have:

P = 2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 10 × . . . × 99 = 2 × 2 × 2 × 3 × 3 × 3 × 3 × 3 × 4 × . . . × 10 Note: m increases by 1 when m 1 is a perfect square. = 2 4 1 3 9 4 4 16 9 . . . k k 2 ( k 1 ) 2 . . . 1 0 99 81 = 2 3 3 5 4 7 5 9 6 11 7 13 8 15 9 17 1 0 18 = 2 3 ( 2 2 ) 7 ( 2 11 3 11 ) ( 2 3 ) 15 ( 2 18 5 18 ) 3 5 5 9 7 13 9 17 = 2 3 + 14 + 11 + 45 + 18 3 16 5 9 7 13 9 17 = 2 91 3 16 5 9 7 13 9 17 \begin{aligned} P & = \left \lceil \sqrt{2} \right \rceil \times \left \lceil \sqrt{3} \right \rceil \times \left \lceil \sqrt{\color{#3D99F6}{4}} \right \rceil \times \left \lceil \sqrt{5} \right \rceil \times \left \lceil \sqrt{6} \right \rceil \times \left \lceil \sqrt{7} \right \rceil \times \left \lceil \sqrt{8} \right \rceil \times \left \lceil \sqrt{\color{#D61F06}{9}} \right \rceil \times \left \lceil \sqrt{10} \right \rceil \times ... \times \left \lceil \sqrt{99} \right \rceil \\ & = 2 \times 2 \times \color{#3D99F6}{2} \times 3 \times 3 \times 3 \times 3 \times \color{#D61F06}{3} \times 4 \times ... \times 10 \quad \quad \small \color{#3D99F6}{\text{Note: }\left \lfloor \sqrt{m} \right \rfloor \text{ increases by 1 when } m-1 \text{ is a perfect square.}} \\ & = 2^{4-1}3^{9-4}4^{16-9}...k^{k^2-(k-1)^2}...10^{99-81} \\ & = \color{#3D99F6}{2^3}3^5\color{#D61F06}{4^7}5^{9}\color{#3D99F6}{6^{11}}7^{13} \color{#D61F06}{8^{15}}9^{17}\color{#3D99F6}{10^{18}} \\ & = \color{#3D99F6}{2^3} \color{#D61F06}{(2^2)^7} \color{#3D99F6}{(2^{11}3^{11})} \color{#D61F06}{(2^3)^{15}} \color{#3D99F6}{(2^{18}5^{18})} 3^5 5^{9}7^{13} 9^{17} \\ & = 2^{\color{#3D99F6}{3}+ \color{#D61F06}{14}+ \color{#3D99F6}{11} + \color{#D61F06}{45}+\color{#3D99F6}{18}} 3^{16} 5^{9}7^{13} 9^{17} \\ & = 2^{\color{#3D99F6}{91}} 3^{16} 5^{9}7^{13} 9^{17} \end{aligned}

n = 91 \Rightarrow n = \boxed{\color{#3D99F6}{91}} .

Nice solution! +1.

The question asks for generalization. Although I have made some progress to generalize the solution, I couldn't yet figure out how to do this. Any ideas?

Arulx Z - 5 years, 6 months ago

Did the same

Shreyash Rai - 5 years, 6 months ago

Suppose that the expression is equal to A. Then, A = 2 3 × 3 5 × 4 7 × 5 9 × 6 11 × 7 13 × 8 15 × 9 17 × 1 0 18 A = 2^{3} \times 3^{5} \times 4^{7} \times 5^{9} \times 6^{11} \times 7^{13} \times 8^{15} \times 9^{17} \times 10^{18} Asked : The largest non-negative integer n such that 2 n 2^{n} is a factor of A So, n = 3 + 2 × 7 + 11 + 3 × 15 + 18 n = 3 + 2 \times 7 + 11 + 3 \times 15 + 18 Then, n = 91 n = 91

Ununderstandable!!!!! Solution!!!?

ashutosh yadav - 5 years, 6 months ago

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W e f i n d h o w m a n y N g i v e s 2 , 4 , 6 , 8 , b u t l i m i t i s 99 . 1 2 + 1 = 2........... t o . . . . . . . . 2 2 = 4...... 4 2 + 1 = 3 o f 2. 3 2 + 1 = 10........... t o . . . . . . . . 4 2 = 16..... 16 10 + 1 = 7 o f 2 2 . 5 2 + 1 = 26........... t o . . . . . . . . 6 2 = 36..... 36 26 + 1 = 11 o f 2 3. 7 2 + 1 = 50........... t o . . . . . . . . 8 2 = 64..... 64 50 + 1 = 15 o f 2 3 . 9 2 + 1 = 82........... t o . . . . . . . 1 0 2 = 100..... 100 16 + 1 = 19 o f 2 5. S o f r o m 1 t o 100 , w e h a v e n = 3 + 2 7 + 11 + 3 15 + 19 = 92. B u t o u r l i m i t i s u p t o 99 , w h i l e w e h a v e i n c l u d e d 100. S o f r o m s q u a r e r o o t o f 1 t o 99 w e h a v e 2 92 1 . n = 91. We~~ find ~how~ many~~ \lceil \sqrt{N}\rceil~~gives~ 2, 4, 6, 8 , ~~but ~limit~is~ \sqrt{99}.\\ 1^2+1~=~~~2...........to........2^2~=~~4......~~~4-2+1 ~~=~ 3 ~~of~~~~~~2. \\ 3^2+1~=~~10...........to........4^2~=~16.....~~16-10+1 ~~=~7 ~~of~~~~2^2. \\ 5^2+1~=~~26...........to........6^2~=~36.....~~36-26+1 ~~ =11 ~~of~~~~2*3. \\ 7^2+1~=~~50...........to........8^2~=~64.....~64-50+1~~ =15 ~~of~~~~2^3. \\ 9^2+1~=~~82...........to.......10^2~=100.....~~100-16+1~~ =19 ~~of~~~~2*5. \\ ~~~~~~\\ So~from~\sqrt1~~to~~\sqrt{100},~~we~have~~n~=~3+2*7+11+3*15+19=92.\\ ~~~~~\\ But~our~limit~is~upto~99,~while~we~have~included~100. \\ So~from~square~root~of~1~to~99~ we~have~2^{92-1}.~~\implies~~n=\Large~~\color{#D61F06}{91}.

Niranjan Khanderia - 3 years, 1 month ago

I solved it too this way but you need to detail it with all steps.

Shubham Bhargava - 5 years, 6 months ago
Pranav Rao
Dec 6, 2015

Is there any way to generalize it not only for 2 but for other prime factors too?

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