(x-3)(x+1)=15
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(x-3)(x+1)=15
x-3=15 hoặc x+1=15
x=15+3 hoặc x=15-1
x=18 hoặc x=14
có gì sai sót mong mọi người chiếu cố
(x-3)(x+1)=15
=>\(x^2+x-3x-3=15\)
=>\(x^2-2x-18=0\)
=>\(x^2-2x+1-19=0\)
=>\(\left(x-1\right)^2=19\)
=>\(\left[{}\begin{matrix}x-1=\sqrt{19}\\x-1=-\sqrt{19}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\sqrt{19}+1\\x=-\sqrt{19}+1\end{matrix}\right.\)
-10 < x ≤ 13
⇒ x ∈ {-9; -8; -7; ...; 7; 8; 9; 10; 11; 12; 13}
Tổng của chúng là:
-9 + (-8) + (-7) + ... + 7 + 8 + 9 + 10 + 11 + 12 + 13
= 10 + 11 + 12 + 13
= 46
Chọn D
12n + 1 = 12n + 18 - 17
= 6(2n + 3) - 17
Để (12n + 1) ⋮ (2n + 3) thì 17 ⋮ (2n + 3)
⇒ 2n + 3 ∈ Ư(17) = {-17; -1; 1; 17}
⇒ 2n ∈ {-20; -4; -2; 14}
⇒ n ∈ {-10; -2; -1; 7}
Mà n là số tự nhiên
n = 7
b: \(B=16^5+2^{15}\)
\(=\left(2^4\right)^5+2^{15}\)
\(=2^{20}+2^{15}\)
\(=2^{15}\left(2^5+1\right)=2^{15}\cdot33⋮33\)
c: \(45⋮9;99⋮9;180⋮9\)
Do đó: \(45+99+180⋮9\)
=>\(C⋮9\)
d: \(D=2+2^2+2^3+...+2^{60}\)
\(=\left(2+2^2+2^3\right)+\left(2^4+2^5+2^6\right)+...+\left(2^{58}+2^{59}+2^{60}\right)\)
\(=2\left(1+2+2^2\right)+2^4\cdot\left(1+2+2^2\right)+...+2^{58}\left(1+2+2^2\right)\)
\(=7\left(2+2^4+...+2^{58}\right)⋮7\)
\(D=2+2^2+2^3+...+2^{60}\)
\(=\left(2+2^2+2^3+2^4\right)+\left(2^5+2^6+2^7+2^8\right)+...+\left(2^{57}+2^{58}+2^{59}+2^{60}\right)\)
\(=2\left(1+2+2^2+2^3\right)+2^5\left(1+2+2^2+2^3\right)+...+2^{57}\left(1+2+2^2+2^3\right)\)
\(=15\left(2+2^5+...+2^{57}\right)\)
=>D chia hết cho cả 3 và 5
a: A=1+2-3-4+5+6-7-8+...+97+98-99-100
=(1+2-3-4)+(5+6-7-8)+...+(97+98-99-100)
\(=\left(-4\right)+\left(-4\right)+...+\left(-4\right)\)(25 số -4)
\(=-4\cdot25=-100\)
b: \(B=1+3+3^2+...+3^{11}\)
\(=\left(1+3\right)+\left(3^2+3^3\right)+...+\left(3^{10}+3^{11}\right)\)
\(=\left(1+3\right)+3^2\left(1+3\right)+...+3^{10}\left(1+3\right)\)
\(=4\left(1+3^2+...+3^{10}\right)⋮4\)
\(\dfrac{9}{14}+\dfrac{8}{21}=\dfrac{27}{42}+\dfrac{16}{42}=\dfrac{43}{42}\)
\(\dfrac{13}{15}-\dfrac{7}{12}=\dfrac{52}{60}-\dfrac{35}{60}=\dfrac{17}{60}\)
Ta có: \(2^2+2^3+2^4+2^5\)
\(=\left(2^2+2^3\right)+\left(2^4+2^5\right)\)
\(=12+2^2.\left(2^2+2^3\right)\)
\(=12+2^2.12\)
\(=12.\left(1+2^2\right)\)
Vì \(12⋮3\) nên \(12.\left(1+2^2\right)⋮3\)
Vậy \(2^2+2^3+2^4+2^5⋮3\)
X2+X-3X-3=15
X2-2X-3=15
X2-2X=18
X.X-2X=18
X(X-2)=18
TH1
X=18
TH2
X-2=18
X=20
Vậy Xϵ{18;20}
(\(x\) - 3).(\(x\) + 1) = 15
\(x^2\) - 3\(x\) + \(x\) - 3 = 15
\(x^2\) - 2\(x\) - 3 + 4 = 15 + 4
\(x^2\) - 2\(x\) + 1 = 19
(\(x\) - 1)2 = 19
\(\left[{}\begin{matrix}x-1=-\sqrt{19}\\x-1=\sqrt{19}\end{matrix}\right.\)
\(\left[{}\begin{matrix}x=1-\sqrt{19}\\x=1+\sqrt{19}\end{matrix}\right.\)