Hãy tìm lời giải và cách mở rộng cho bài toán trên.
Edited by Phạm Quang Toàn, 31-07-2011 - 10:51.
Edited by Phạm Quang Toàn, 31-07-2011 - 10:51.
Discovery is a child’s privilege. I mean the small child, the child who is not afraid to be wrong, to look silly, to not be serious, and to act differently from everyone else. He is also not afraid that the things he is interested in are in bad taste or turn out to be different from his expectations, from what they should be, or rather he is not afraid of what they actually are. He ignores the silent and flawless consensus that is part of the air we breathe – the consensus of all the people who are, or are reputed to be, reasonable.
Grothendieck, Récoltes et Semailles (“Crops and Seeds”).
liệu có nhầm đề ko bạn. nếu theo đề này thì a = b = c, mak a là số nguyên tố, b chia hết 3 vậy a = b = c = 3!??????Tìm tất cả các bộ ba số tự nhiên $a,b,c$ nhỏ hơn $20$ thỏa mãn điều kiện $a(a+1)=b(b+1)=c(c+1)$, trong đó $a$ là số nguyên tố và $b$ chia hết cho $3$.
Hãy tìm lời giải và cách mở rộng cho bài toán trên.
Tìm tất cả các bộ ba số tự nhiên $a,b,c$ nhỏ hơn $20$ thỏa mãn điều kiện $a(a+1)=b(b+1)=c(c+1)$, trong đó $a$ là số nguyên tố và $b$ chia hết cho $3$.
Hãy tìm lời giải và cách mở rộng cho bài toán trên.
KHÔ”NG THỬ SAO BIẾT!!!
Edited by xusinst, 31-07-2011 - 11:21.
Discovery is a child’s privilege. I mean the small child, the child who is not afraid to be wrong, to look silly, to not be serious, and to act differently from everyone else. He is also not afraid that the things he is interested in are in bad taste or turn out to be different from his expectations, from what they should be, or rather he is not afraid of what they actually are. He ignores the silent and flawless consensus that is part of the air we breathe – the consensus of all the people who are, or are reputed to be, reasonable.
Grothendieck, Récoltes et Semailles (“Crops and Seeds”).
Cho mình xin lỗi, đề là $a(a+1)+b(b+1)=c(c+1)$
KHÔNG THỬ SAO BIẾT!!!
Edited by xusinst, 31-07-2011 - 11:55.
tớ thấy đoạn t+1=bVt+1=2b có vẻ ko đc hợp lí cho lắm vì b ko phải là số nguyên tốMình làm thế này. Các bạn kiểm tra giúp.
Trước hết, ta giải phương trình: $a(a + 1) + b(b + 1) = k(k + 1)\,\,\,\,\,(1)$ với $a,b \in P,\,k \in N$. Ta có:
$(1) \Leftrightarrow a(a + 1) = k(k + 1) - b(b + 1) = (k - b)(k + b + 1) \Rightarrow \left. a \right|\,k - b\, \vee \,\left. a \right|k + b + 1$
Nếu $\left. a \right|\,k - b \Rightarrow a \le k - b \Rightarrow ... \Rightarrow k + b + 1 < k - b + 1$, vô lí
Do đó: $\left. a \right|k + b + 1 \Leftrightarrow k + b + 1 = ta,\,\,t > 1$
$\Rightarrow a + 1 = t(k - b) \Rightarrow a = t(k - b) - 1$
Ta lại có: $2b = (k + b) - (k - b) = (ta - 1) - (k - b) = t\left[ {t(k - b) - 1} \right] - 1 - (k - b)$
$\Leftrightarrow 2b = (t + 1)\left[ {(t - 1)(k - b) - 1} \right]\,\,\,(2)$
Ta có: t+1>2. Do đó ta có: $\left[ \begin{array}{l}t + 1 = b \\ t + 1 = 2b \\ \end{array} \right.$
* Nếu t+1=b thì (t-1)(k-b)=3 $\Rightarrow \left[ \begin{array}{l}t - 1 = 1 \\ t - 1 = 3 \\ \end{array} \right. \Rightarrow \left[ \begin{array}{l}t = 2 \\ t = 4 \\ \end{array} \right.$
t=2, ta có: $b = 3 \Rightarrow k - 3 = 3 \Rightarrow k = 6,\,a = 5$
t=4, ta có: $b = 5 \Rightarrow k - 5 = 1 \Rightarrow k = 6,\,a = 3$
* Nếu t+1=2b thì (t-1)(k-b)=2 $\Rightarrow t - 1 = 1,\,k - b = 2 \Rightarrow t = 2,\,b \notin N$
hoặc $t - 1 = 2,\,k - b = 1 \Rightarrow t = 3,\,b = 2,k = 3 \Rightarrow a = 2$.
Do đó pt (1) có các nghiệm nguyên dương là: $\left[ \begin{array}{l}a = 5;\,\,b = 3;\,\,k = 6 \\ a = 3;\,\,b = 5;\,\,k = 6 \\ a = 2;\,\,b = 2;\,\,k = 3 \\ \end{array} \right.$
Theo ycbt thì ta được a=5, b=3, c=6.
P/s: Bài này là 1 trường hợp của bài giải phương trình mà mình đã post ở trên.
------------------------KHÔNG THỬ SAO BIẾT!!!
Discovery is a child’s privilege. I mean the small child, the child who is not afraid to be wrong, to look silly, to not be serious, and to act differently from everyone else. He is also not afraid that the things he is interested in are in bad taste or turn out to be different from his expectations, from what they should be, or rather he is not afraid of what they actually are. He ignores the silent and flawless consensus that is part of the air we breathe – the consensus of all the people who are, or are reputed to be, reasonable.
Grothendieck, Récoltes et Semailles (“Crops and Seeds”).
Edited by Phạm Quang Toàn, 07-08-2011 - 21:45.
Discovery is a child’s privilege. I mean the small child, the child who is not afraid to be wrong, to look silly, to not be serious, and to act differently from everyone else. He is also not afraid that the things he is interested in are in bad taste or turn out to be different from his expectations, from what they should be, or rather he is not afraid of what they actually are. He ignores the silent and flawless consensus that is part of the air we breathe – the consensus of all the people who are, or are reputed to be, reasonable.
Grothendieck, Récoltes et Semailles (“Crops and Seeds”).
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