3/10

1275

D= \(\frac{3}{4.5}+\frac{3}{5.6}+\frac{3}{6.7}+\cdots+\frac{3}{93.94}+\frac{3}{94.95}\) ​

\(= 3 \times \left(\right. \frac{1}{4 \times 5} + \frac{1}{5 \times 6} + \frac{1}{6 \times 7} + . . . + \frac{1}{93 \times 94} + \frac{1}{94 \times 95} \left.\right)\)

\(= 3 \times \left(\right. \frac{5 - 4}{4 \times 5} + \frac{6 - 5}{5 \times 6} + \frac{7 - 6}{6 \times 7} + . . . + \frac{94 - 93}{93 \times 94} + \frac{95 - 94}{94 \times 95} \left.\right)\)

\(= 3 \times \left(\right. \frac{1}{4} - \frac{1}{5} + \frac{1}{5} - \frac{1}{6} + \frac{1}{6} - \frac{1}{7} + . . . + \frac{1}{93} - \frac{1}{94} + \frac{1}{94} - \frac{1}{95} \left.\right)\)

\(= 3 \times \left(\right. \frac{1}{4} - \frac{1}{95} \left.\right)\)

\(= 3 \times \frac{91}{380}\)

\(= \frac{273}{380}\)

1x2= 2            2x3= 6           3x4= 12               4x5= 20

5x6= 30            6x7= 42           7x8= 56               8x9= 72

1x2=?            2x3=?           3x4=?               4x5=?

5x6=?            6x7=?           7x8=?               8x9=

b. chữ số 4 có giá trị là phần mười