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

(x-7).(x-7)=(-2).(-18)

\((x-7)^2\) =36

\((x-7)^2\) =\((\pm6)^2\)

TH1:x-7=6 TH2:x-7=(-6)

x =6+7 x =(-6)+7

x =13 x =1

vậy x∈{13;1}

\(36^2=36.36=1296\)

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80}<1\)

⇒A\(<1\)

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80} < \frac{1}{9}\)

⇒A\(< \frac{1}{9}\)

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80} < \frac{1}{9}\)

⇒A\(< \frac{1}{9}\)

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80}<\frac19\)

⇒A\(<\frac19\)

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80}<\frac18\)

⇒A\(<\frac18\)

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80} < 1\)

⇒A<1

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80} < 1\)

⇒A<1

A=\(3 \left(\right. \frac{3}{20.23} + \frac{3}{23.26} + \frac{3}{26.29} + . . . + \frac{3}{77.80} \left.\right)\)

A\(= 3 \left(\right. \frac{1}{20} - \frac{1}{23} + \frac{1}{23} - \frac{1}{26} + \frac{1}{26} - \frac{1}{29} + . . . + \frac{1}{77} - \frac{1}{80} \left.\right)\)

\(A = 3 \left(\right. \frac{1}{20} - \frac{1}{80} \left.\right)\)

A\(= 3 \left(\right. \frac{4}{80} - \frac{1}{80} \left.\right)\)

A\(= 3. \frac{3}{80}\)

A\(= \frac{9}{80}\)

\(\frac{9}{80} < 1\)

⇒A<1