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?$?解：要使分式\frac 1 {4x}有意義，$?
?$則4x≠0$?
?$解得，x≠0$?
?$∴ 當(dāng)x≠0時(shí)，分式\frac 1 {4x}有意義。$?
?$?解：要使分式\frac {3x+1} {3-7x}有意義，$?
?$則3-7x≠0$?
?$解得，x≠\frac 3 7$?
?$∴ 當(dāng)x≠\frac 3 7時(shí)，分式\frac {3x+1} {3-7x}有意義。$?
?$?解：要使分式\frac {x+1} {{(2x+1)}^{2}}有意義，$?
?$則2x+1≠0，$?
?$解得，x≠-{\frac {1} {2}}$?
?$∴ 當(dāng)x≠-{\frac {1} {2}}時(shí)，分式\frac {x+1} {{(2x+1)}^{2}}有意義。$?
?$解：要使分式\frac {1} {(x-1)(2x+4)}有意義，$?
?$則(x-1)(2x+4)≠0$?
?$解得，x≠1且x≠-2$?
?$∴ 當(dāng)x≠1且x≠-2時(shí)，分式$?
?$\frac {1} {(x-1)(2x+4)}有意義。$?
?$?解：依題意得，$?
?${{\begin{cases} {{a+5=0}} \\ {{a}^{2}≠0} \end{cases}}}$?
?$解得，a=-5$?
?$∴ 當(dāng)a=-5時(shí)，分式的值為0$?
?$?解：依題意得，$?
?${{\begin{cases} {{2a-1=0}} \\ {a+2≠0} \end{cases}}}$?
?$解得，a={\frac {1} {2}}$?
?$∴ 當(dāng)a={\frac {1} {2}}時(shí)，分式的值為0$?
?$?解：依題意得，$?
?${{\begin{cases} {{{a}^{2}-4=0}} \\ {{a}^{2}+2≠0} \end{cases}}}$?
?$解得，a=\pm 2$?
?$∴ 當(dāng)a=\pm 2時(shí)，分式的值為0$?
?$解： 依題意得，$?
?${{\begin{cases} {{\left | {{a}} \right |-1=0}} \\ {a-1≠0} \end{cases}}}$?
?$解得，a=-1$?
?$∴ 當(dāng)a=-1時(shí)，分式的值為0$?
B
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