Dạng 6. Tìm số chưa biết trong một tổng, một hiệu có đáp án

a) $2\frac{2}{3}.x+8\frac{2}{3}=3\frac{1}{3}$

$8.x+26=10$

$8.x=10-26$

$8.x=-16$

$x=-2$

b) $3\frac{2}{7}.x-\frac{1}{8}=2\frac{3}{4}$

$\frac{23}{7}.x-\frac{1}{8}=\frac{11}{4}$

$\frac{23}{7}.x=\frac{11}{4}+\frac{1}{8}$

$\frac{23}{7}.x=\frac{23}{8}$

$x=\frac{23}{8}:\frac{23}{7}$

$x=\frac{23}{8}.\frac{7}{23}$

a)$3\frac{2}{3}.x+5\frac{1}{2}=11\frac{1}{6}$$3\frac{2}{3}.x=11\frac{1}{6}-5\frac{1}{2}$$3\frac{2}{3}.x=\frac{67}{6}-\frac{11}{2}$$3\frac{2}{3}.x=\frac{17}{3}$$x=\frac{17}{3}:3\frac{2}{3}$$x=\frac{17}{3}:\frac{11}{3}$$x=\frac{17}{11}$

$b)$$2\frac{3}{5}.x-\frac{1}{7}=1\frac{9}{35}$

$2\frac{3}{5}.x=1\frac{9}{35}+\frac{1}{7}$

$2\frac{3}{5}.x=\frac{44}{35}+\frac{1}{7}$

$2\frac{3}{5}.x=\frac{7}{5}$

$x=\frac{7}{5}:2\frac{3}{5}$

$x=\frac{7}{5}:\frac{13}{5}$

$x=\frac{7}{13}$

a) $\left(2\frac{4}{5}.x+50\right):\frac{2}{3}=-51$

$2\frac{4}{5}.x+50=-51.\frac{2}{3}$

$2\frac{4}{5}.x+50=-34$

$2\frac{4}{5}.x=-34-50$

$2\frac{4}{5}.x=-84$

$x=-84:2\frac{4}{5}$

$x=-84:\frac{14}{5}$

$x=-30$

b) $\left(4\frac{1}{2}-2x\right).1\frac{4}{61}=6\frac{1}{2}$

$\left(\frac{9}{2}-2x\right).\frac{65}{61}=\frac{13}{2}$

$\frac{9}{2}-2x=\frac{13}{2}:\frac{65}{61}$

$\frac{9}{2}-2x=\frac{61}{10}$

$2x=\frac{9}{2}-\frac{61}{10}$

$2x=\frac{-8}{5}$

$x=\frac{-8}{5}:2$

$x=\frac{-4}{5}$

$3,2.x-\left(\frac{4}{5}+\frac{2}{3}\right):3\frac{2}{3}=\frac{7}{20}$

$3,2.x-\frac{22}{15}:\frac{11}{3}=\frac{7}{20}$

$3,2.x-\frac{2}{5}=\frac{7}{20}$

$3,2.x=\frac{7}{20}+\frac{2}{5}$

$3,2.x=\frac{3}{4}$

$x=\frac{3}{4}:3,2$

$x=\frac{15}{64}$