Bài 1: tính nhanh
a. 17262 – 9998 = 17262 – 9998 – 2 + 2 = 17264 – 10000 = 7264
b. 846 – 197 = 846 – 197 + 4 – 3 - 1 = (846 + 4) – (197 + 3) – 1 = 850 -200 – 1 = 650 – 1 = 649
c. 202020202019 – 201920192020 = 1 + 202020202019 – 201920192020 - 1
= 202020202020 – 201920192019 = 2020.100010001 – 2019.100010001
= 100010001.( 2020 – 2019) = 100010001
d. 100 + 98 + 96 +…+ 4 + 2 - 97 – 95 -… - 3 – 1
=(100 + 98 + 96 +…+ 4 + 2) – (97 + 95 + … + 3 + 1)
$=\left( \frac{100-2}{2}+1 \right).\frac{100+2}{2}-\left( \frac{97-1}{2}+1 \right).\frac{97+1}{2}$
$=2550-2401$ = 149
e. $425.7.4-170.60$$=425.4.7-170.10.6$$=1700.7-1700.6$$=1700.\left( 7-6 \right)$$=1700$

bài 2 em xem lại đề bài nhé