\(\dfrac{1}{\sqrt{\dfrac{5}{7}}+\sqrt{\dfrac{5}{13}}+1}+\dfrac{1}{\sqrt{\dfrac{7}{13}}+\sqrt{\dfrac{7}{5}}+1}+\dfrac{1}{\sqrt{1\dfrac{6}{7}}+\sqrt{2\dfrac{3}{5}}+1}\\ =\dfrac{1}{\dfrac{\sqrt{5}}{\sqrt{7}}+\dfrac{\sqrt{5}}{\sqrt{13}}+\dfrac{\sqrt{5}}{\sqrt{5}}}+\dfrac{1}{\dfrac{\sqrt{7}}{\sqrt{13}}+\dfrac{\sqrt{7}}{\sqrt{5}}+\dfrac{\sqrt{7}}{\sqrt{7}}}+\dfrac{1}{\dfrac{\sqrt{13}}{\sqrt{7}}+\dfrac{\sqrt{13}}{\sqrt{5}}+\dfrac{\sqrt{13}}{\sqrt{13}}}\\ =\left(\dfrac{1}{\sqrt{5}}+\dfrac{1}{\sqrt{7}}+\dfrac{1}{\sqrt{13}}\right)\cdot\dfrac{1}{\dfrac{1}{\sqrt{5}}+\dfrac{1}{\sqrt{7}}+\dfrac{1}{\sqrt{13}}}\\ =1\)

Làm giúp mik bài 2 vs 4 với ạ pls

a, \(\left(-17\right)+5+8+17+\left(-3\right)\)

\(=\left(-17+17\right)+\left[5+\left(-3\right)\right]+8\)

\(=0+8+8=8+8=16\)

b, \(\left(5^{19}:5^{17}+3\right):7=\left(5^2+3\right):7\)

\(=\left(25+3\right):7=28:7=4\)

c, \(|-8|+\left(-5\right)+9+\left(-7\right)+|-4|\)

\(=8-5+9-7+4=3+2+4=5+4=9\)

ý d mk ko biết nha.

thông cảm cho mk nha.

k mk nha.

#mon

a.

Đặt \(cos2x=t\Rightarrow t\in\left[-1;1\right]\)

Xét hàm \(y=f\left(t\right)=2t^2+2t-4\) trên \(\left[-1;1\right]\)

\(-\dfrac{b}{2a}=-\dfrac{1}{2}\in\left[-1;1\right]\)

\(f\left(-1\right)=-4\) ; \(f\left(-\dfrac{1}{2}\right)=-\dfrac{9}{2}\) ; \(f\left(1\right)=0\)

\(\Rightarrow y_{min}=-\dfrac{9}{2}\) khi \(t=-\dfrac{1}{2}\) hay \(cos2x=-\dfrac{1}{2}\)

\(y_{max}=0\) khi \(cos2x=1\)

b. Đặt \(tanx=t\Rightarrow t\in\left[-1;\sqrt{3}\right]\)

Xét hàm \(f\left(t\right)=t^2-2\sqrt{3}t-1\) trên \(\left[-1;\sqrt{3}\right]\)

\(-\dfrac{b}{2a}=\sqrt{3}\in\left[-1;\sqrt{3}\right]\)

\(f\left(-1\right)=2\sqrt{3}\) ; \(f\left(\sqrt{3}\right)=-4\)

\(y_{min}=-4\) khi \(x=\dfrac{\pi}{3}\) ; \(y_{max}=2\sqrt{3}\) khi \(x=-\dfrac{\pi}{4}\)

\(\left(x+\frac{4}{3}\right)\cdot\frac{7}{4}=5-\frac{7}{6}\)

\(\left(x+\frac{4}{3}\right)\cdot\frac{7}{4}=\frac{23}{6}\)

\(x+\frac{4}{3}=\frac{23}{6}:\frac{7}{4}=\frac{46}{21}\)

\(x=\frac{46}{21}-\frac{4}{3}\)

\(x=\frac{6}{7}\)

Vậy ....................................

1*2*3*4*5*6+711 không chia hết cho 2

vì 711 không chia hết cho 2

1*2*3*4*5*6+711

711 chia hết cho 9

1*2*3*4*5*6=1*2*4*5*(3*6)

                   =1*2*4*5*18

vì 18 chia hết cho 2

=>1*2*3*4*5*6+711 chia hết cho 2

ở cuối chia hết cho 9,thông cảm

Tổng 1 . 2 . 3 . 4 . 5 . 6 + 711 không chia hết cho 2 nhưng chia hết cho 9 vì :

 1 . 2 . 3 . 4 . 5 . 6 chia hết cho 2

711 không chia hết cho 2

=>1 . 2 . 3 . 4 . 5 . 6 + 711 không chia hết cho 2

1 . 2 . 3 . 4 . 5 . 6 = 1 . 2 . 3 . 4 . 5 . 3 . 2 = 1 . 2 . 4 . 5 . 2 . 9 chia hết cho 9

711 chia hết cho 9

=> 1 . 2 . 3 . 4 . 5 . 6 + 711 chia hết cho 9

Ta có \(E=\frac{5n-4}{2n+5}\)

\(\Rightarrow2E=\frac{10n-8}{2n+5}=\frac{5\left(2n+5\right)-33}{2n+5}=5-\frac{33}{2n+5}\)

Để E nguyên => 2E nguyên => 5-\(\frac{33}{2n+5}\)nguyên

=> \(\frac{33}{2n+5}\)nguyên

=> \(33⋮2n+5\)

\(\Rightarrow2n+5=Ư_{\left(33\right)}=\left\{-33;-1;1;33\right\}\)

Ta có bảng

Vậy n={-19;-3;-2;14}