Đáp án:
$
A\left ( \dfrac{5}{2};\dfrac{-3}{4} \right )=2\\
A\left ( \dfrac{-5}{2};\dfrac{-3}{4} \right )=\dfrac{-1}{2}\\
B=\dfrac{27}{20}$
Giải thích các bước giải:
$|x|=2,5\Rightarrow x=\pm \dfrac{5}{2},y=\dfrac{-3}{4}\\
A\left ( \dfrac{5}{2};\dfrac{-3}{4} \right )=2x+2xy-y=2.\dfrac{5}{2}+2.\dfrac{5}{2}.\dfrac{-3}{4}-\dfrac{-3}{4}\\
=5+5.\dfrac{-3}{4}+\dfrac{3}{4}\\
=5-\dfrac{15}{4}+\dfrac{3}{4}\\
=5-\dfrac{12}{4}\\
=5-3\\
=2\\
A\left ( \dfrac{-5}{2};\dfrac{-3}{4} \right )=2x+2xy-y=2.\dfrac{-5}{2}+2.\dfrac{-5}{2}.\dfrac{-3}{4}-\dfrac{-3}{4}\\
=-5+5.\dfrac{3}{4}+\dfrac{3}{4}\\
=-5+\dfrac{15}{4}+\dfrac{3}{4}\\
=-5+\dfrac{18}{4}\\
=\dfrac{-20}{4}+\dfrac{18}{4}\\
=\dfrac{-2}{4}\\
=\dfrac{-1}{2}\\
B=\dfrac{3}{2.3}+\dfrac{3}{3.4}+\dfrac{3}{4.5}+...+\dfrac{3}{19.20}\\
=3.\left ( \dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{19.20}\right )\\
=3.\left ( \dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{19}-\dfrac{1}{20} \right )\\
=3.\left ( \dfrac{1}{2}-\dfrac{1}{20} \right )\\
=3.\left ( \dfrac{10}{20}-\dfrac{1}{20} \right )\\
=3.\dfrac{9}{20}\\
=\dfrac{27}{20}$
