b: \(=\dfrac{2}{7}-\dfrac{3}{7}\cdot\dfrac{-2}{3}=\dfrac{2}{7}+\dfrac{2}{7}=\dfrac{4}{7}\)

c: \(=\dfrac{3}{7}-\dfrac{7}{2}-\dfrac{3}{7}+\dfrac{7}{2}=0\)

Bài 1: 

a, \(\dfrac{-x-2}{3}\) = - \(\dfrac{6}{7}\)

      - \(x\) - 2 = - \(\dfrac{18}{7}\)

         \(x\)      = - 2 + \(\dfrac{18}{7}\)

         \(x\)      = - \(\dfrac{4}{7}\)

Bài b,  \(\dfrac{4}{7-x}\) = \(\dfrac{1}{3}\)

            12 = 7 - \(x\)

            \(x\)  = 7 - 12 

            \(x\)  = -5 

\(\Rightarrow\dfrac{5}{7}-x=-\dfrac{1}{5}\Rightarrow x=\dfrac{32}{35}\)

a) \(\frac{1}{3}-\frac{3}{4}-\left(\frac{-3}{5}\right)+\frac{1}{64}-\frac{2}{9}-\frac{1}{36}+\frac{1}{15}\)

=\(\left(\frac{1}{3}+\frac{3}{5}+\frac{1}{15}\right)-\left(\frac{3}{4}+\frac{2}{9}+\frac{1}{36}\right)+\frac{1}{64} \)

=\(\frac{5+9+1}{15}-\frac{27+8+1}{36}+\frac{1}{64}\)

= \(1+1+\frac{1}{64}=2\frac{1}{64}\)

\(16\frac{2}{7}:\left(\frac{-3}{5}\right)+28\frac{2}{7}:\frac{3}{5}\)

=\(\frac{114}{7}:\left(\frac{-3}{5}\right)+\frac{198}{7}:\frac{3}{5}\)

=\(-\frac{190}{7}+\frac{330}{7}\)

=\(\frac{-190+330}{7}=\frac{140}{7}=20\)

\(\frac{1}{5}A=\frac{1}{5}.\left(\frac{1}{5}+\frac{1}{5^2}+...+\frac{1}{5^{20}}\right)\)

\(\Rightarrow\frac{1}{5}A=\frac{1}{5^2}+\frac{1}{5^3}+...+\frac{1}{5^{20}}\)

\(\Rightarrow\frac{1}{5}A-A=\left(\frac{1}{5^2}+...+\frac{1}{5^{21}}\right)-\left(\frac{1}{5}+...+\frac{1}{5^{20}}\right)\)

\(-\frac{4}{5}A=\frac{1}{5^{21}}-\frac{1}{5}\)

\(\Rightarrow A=\left(\frac{1}{5^{21}}-\frac{1}{5}\right):\left(-\frac{4}{5}\right)\)

các câu còn lại tương tự thôi

B1 c2

dùng xích ma \(\text{∑}^{20}_1\left(\frac{1}{5^x}\right)=0,25=\frac{1}{4}\)

chỗ phía dưới là 1 nha nó bị che

cậu giải thích giùm mình đoạn này với P(x)=x^7-(x+1)x^6+(x+1)x^5-(x+1)x^4+(x+1)x^3-(x+1)x^2+(x+1)x+15

P(x)=x^7-x^7-x^6+x^6+x^5-x^5-x^4+x^4+x^3-x^3-x^2+x^2+x+15

P(x)=x+15=79+15=94

hay giai giup mk may phan nay nhe

cmr cac bieu thuc sau ko phu thuoc vao x:

c)C=x(x^3+x^2-3x-2)-(x^2-2)(x^2+x-1)

e)E=(x+1)(x^2-x+1)-(x-1)(x^2+x+1)

tinh gia tri cua da thuc

b)Q(x)=x^14-10x^13=10x^12-10x^11+...+10x^2-10x+10   voi x=9

c)R(x)=x^4-17x^3+17x^2_17x+20   või=16

d)S(x)=x^10-13x^9+13x^8-13X^7+...+13x^2-13x+10     voi 12

a: \(=\left(\dfrac{5}{8}-\dfrac{28}{8}\right)\cdot\dfrac{5}{6}=\dfrac{-23}{8}\cdot\dfrac{5}{6}=\dfrac{-115}{48}\)

b: \(=\dfrac{4}{7}\cdot\dfrac{24+25}{15}=\dfrac{4}{7}\cdot\dfrac{49}{15}=\dfrac{28}{15}\)

c: \(=\dfrac{7}{2}:\dfrac{24-10}{15}=\dfrac{7}{2}\cdot\dfrac{15}{14}=\dfrac{15}{4}\)

bn oi tính bằng 2 cách ạ