Đặt \(A=2,7{c^2} - 3,5c\)
+ Với \(c = 0,7\) ta có:
\(A = 2,7.{\left( {0,7} \right)^2} - 3,5.0,7\)\(\, = 2,7.0,49 - 3,5.0,7 \)\(\,= 1,323 - 2,45 = - 1,127\)
+Với \(c = \dfrac{2}{3}\) ta có:
\(A = 2,7.{\left( {\dfrac{2}{3}} \right)^2} - 3,5.\left( {\dfrac{2}{3}} \right) \)
\( = 2,7.\dfrac{4}{9} - 3,5.\dfrac{2}{3}\)
\( = \dfrac{{10,8}}{9} - \dfrac{7}{3} = \dfrac{{10,8 - 21}}{9} \)
\(= \dfrac{{ - 10,2}}{9} = \dfrac{{ - 17}}{{15}}\)
+ Với \(c = 1\dfrac{1}{6} = \dfrac{7}{6}\), ta có:
\(A = 2,7.{\left( {\dfrac{7}{6}} \right)^2} - 3,5.\left( {\dfrac{7}{6}} \right)\)
\( = 2,7.\dfrac{{49}}{{36}} - 3,5.\dfrac{7}{6}\)
\( = \dfrac{{132,3}}{{36}} - \dfrac{{24,5}}{6} = \dfrac{{132,3 - 147}}{{36}}\)
\( = \dfrac{{ - 14,7}}{{36}} = \dfrac{{ - 4,9}}{{12}} = \dfrac{{ - 49}}{{120}}\)
