第15页 - 亮点给力提优课时作业本七年级数学江苏版

$6\ $

$\frac {n(n+1)·(－1)^{n}}{3^{n}}$

67

$ \begin{aligned}解：原式&=8-10-2+5 \\ &=1 \\ \end{aligned}$

$ \begin{aligned}解：原式&=(-2)×3×(－3) \\ &=18 \\ \end{aligned}$

$ \begin{aligned}解：原式&=(－\frac{1}{2})×(-24)+\frac{3}{4}×(-24)+ (－\frac{1}{3})×(－24) \\ &=12-18+8 \\ &=2 \\ \end{aligned}$

$ \begin{aligned}解：原式&=9+(20+8)÷(-3) \\ &=9-\frac{28}{3} \\ &=－\frac{1}{3} \\ \end{aligned}$

$解：原式=-3xy^{2}+2xy-x^{2}y-3xy+3xy^{2}$

$=-x^{2}y-xy$

$当x=\frac{3}{4}，y=-1时$

$原式=-(\frac{3}{4})^{2}×(-1)-\frac{3}{4}×(-1)$

$=\frac{9}{16}+\frac{3}{4}$

$=\frac{21}{16}$