1. 计算:
(1) $-2 - 1.6 + (-16) - (-19)$;
(2) $-2^2 × 7 - (-3) × 6 + 5$;
(3) $-\dfrac{3}{4} ÷ \dfrac{3}{8} × (-\dfrac{4}{9}) ÷ (-\dfrac{2}{3})$;
(4) $(\dfrac{7}{9} - \dfrac{5}{6} + \dfrac{5}{18}) × (-18)$;
(5) $(\dfrac{1}{6} - 1\dfrac{1}{3} + 0.75) ÷ (-\dfrac{1}{24})$;
(6) $-2^3 × 0.5 - (-1\dfrac{3}{5})^2 ÷ (-2)^2$.

(1) $-0.6$
(2) $-5$
(3) $-\dfrac{4}{3}$
(4) $-4$
(5) $10$
(6) $-4.64$

2. 化简:
(1)$3a^{2}+2a-\frac{1}{2}a^{2}-\frac{14}{3}a$；
(2)$-2x^{2}-5x+3-3x^{2}+6x-1$；
(3)$3(2x^{2}-y^{2})-2(3y^{2}-2x^{2})$；
(4)$-3(2x^{2}-xy)-(x^{2}+xy-6)-6$。

(1) $\dfrac{5}{2}a^{2}-\dfrac{8}{3}a$
(2) $-5x^{2}+x+2$
(3) $10x^{2}-9y^{2}$
(4) $-7x^{2}+2xy$

3. 先化简,再求值:
$2(3a-5b)-2(-a+b)-3(\dfrac{1}{3}a-\dfrac{5}{6}b)$,其中$a=-\dfrac{2}{7},b=2$.

$2(3a-5b)-2(-a+b)-3 (\dfrac{1}{3}a-\dfrac{5}{6}b ) =6a-10b+2a-2b-a+\dfrac{5}{2}b=(6+2-1)a+ (-10-2+\dfrac{5}{2} ) b=7a-\dfrac{19}{2}b$.
当$a=-\dfrac{2}{7},b=2$时,原式$=7× (-\dfrac{2}{7})-\dfrac{19}{2}× 2=-2-19=-21$.