1. Nam wrote an essay in Literature class morning.
......................................................................................................................................................................................................................................................................................................2. I watched TV yesterday morning.
................................................................................................................................................................................................................................................................................................3. Nam and you were in the English club last Tuesday.
................................................................................................................................................................................................................................................................................................4. They ate noodles two hour ago.
.................................................................................................................................................................................................................................................................................................5. We always had a nice on Christmas holiday in the past.
................................................................................................................................................................................................................................................................................................6. My father decorated the Christmas tree.
...............................................................................................................................................................................................................................................................................................7. She bought a new dress yesterday.
................................................................................................................................................................................................................................................................................................8. They were lat for school.
................................................................................................................................................................................................................................................................................................9. Mr. Tam took his children to the museum last sunday.
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................................................................................................................................................10. Hoa made a cushion for her armchair.
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\(\frac{a+c}{b+d}\) = \(\frac{a-c}{b-d}\)
Áp dụng tính chất dãy tỉ số bằng nhau ta có:
\(\frac{a+c}{b+d}\) = \(\frac{a-c}{b-d}\) = \(\frac{a+c+a-c}{b+d+b-d}\) = \(\frac{\left(a+a\right)+\left(c-c\right)}{\left(b+b\right)+\left(d-d\right)}\) = \(\frac{2a+0}{2b+0}\) = \(\frac{2a}{2b}=\frac{a}{b}\) (1)
\(\frac{a+c}{b+d}=\frac{a-c}{b-d}\)
Áp dụng tính chất dãy tỉ số bằng nhau ta có:
\(\frac{a+c}{b+d}\) = \(\frac{a-c}{b-d}\) = \(\frac{a+c-\left(a-c\right)}{b+d-\left(b-d\right)}\) = \(\frac{a+c-a+c}{b+d-b+d}\) = \(\frac{\left(a-a\right)+\left(c+c\right)}{\left(b-b\right)+\left(d+d\right)}\) = \(\frac{0+2c}{0+2d}\) = \(\frac{c}{d}\)(2)
Kết hợp (1) và (2) ta có: \(\frac{a+c}{b+d}=\frac{a}{b}=\frac{c}{d}\) (đpcm)
E