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第81頁

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解：?$\frac {5x + 1}{6} = \frac {7}{2}$?

?$ $?兩邊乘以?$6$?得：?$5x + 1 = 21$?

移項得：?$5x = 20$?

?$ $?系數化為?$1$?得：?$x = 4$?

解：?$\frac {3 - x}{2} = \frac {x - 4}{3}$?

?$ $?兩邊乘以?$6$?得：?$3(3 - x) = 2(x - 4)$?

去括號得：?$9 - 3x = 2x - 8$?

移項得：?$-5x = -17$?

?$ $?系數化為?$1$?得：?$x = \frac {17}{5}$?

解：?$\frac {x}{2} - \frac {x - 1}{3} = 1$?

?$ $?兩邊乘以?$6$?得：?$3x - 2(x - 1) = 6$?

去括號得：?$3x - 2x + 2 = 6$?

移項得：?$x = 4$?

解：?$\frac {2x - 1}{0.7} + \frac {1}{7} = \frac {x}{0.3}$?

?$ $?兩邊分子分母同乘?$10$?化簡得：?$\frac {20x - 10}{7} + \frac {1}{7} = \frac {10x}{3}$?

?$ $?兩邊乘以?$21$?得：?$3(20x - 10) + 3 = 70x$?

去括號得：?$60x - 30 + 3 = 70x$?

移項得：?$-10x = 27$?

?$ $?系數化為?$1$?得：?$x = -2.7$?

解：根據題意，得?$\frac {x - 7}{5} - 1 = \frac {2x - 1}{2} + 3$?

去分母，兩邊同乘?$10，$?得?$2(x - 7) - 10 = 5(2x - 1) + 30$?

去括號，得?$2x - 14 - 10 = 10x - 5 + 30$?

移項，得?$2x - 10x = -5 + 30 + 14 + 10$?

合并同類項，得?$-8x = 49$?

?$ $?系數化為?$1，$?得?$x = -\frac {49}{8}$?

答：當?$x = -\frac {49}{8}$?時，兩個代數式的值相等。

解：?$(1)$?當?$a = 2，$??$b=\frac {7}{2}，$??$h = 3$?時，

?$ S=\frac {1}{2}(a + b)h=\frac {1}{2}×(2+\frac {7}{2})×3=\frac {1}{2}×\frac {11}{2}×3=\frac {33}{4}$?

?$ (2)$?當?$a = 2，$??$h = 4，$??$S = 16$?時，

?$ 16=\frac {1}{2}(2 + b)×4$?

?$ 16=2(2 + b)$?

?$ 8=2 + b$?

?$ b=6$?

解：原方程可化為：$\frac{10(x + 4)}{2} - \frac{10(x - 3)}{5} = 2$

化簡得：$5(x + 4) - 2(x - 3) = 2$

去括號：$5x + 20 - 2x + 6 = 2$

移項：$5x - 2x = 2 - 20 - 6$

合并同類項：$3x = -24$

系數化為1：$x = -8$

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