Question

Bài 3. Tính nhẩm bằng cách sử dụng các hằng đẳng thức

a) 19²; 28²; 81²; 91²;                           b) 19. 21; 29. 31; 39. 41;          

c) 29² - 8²; 56² - 46²; 67² - 56²;

Answer 1

192=(20−1)2=202−2.20.1+12=400−40+1=361

Answer 2

a)

19^2=(20−1)^2=20^2−2.20.1+1^2=400−40+1=361

28^2=(30−2)^2=30^2−2.30.2+2^2=900−120+4=784

81^2=(80+1)^2=80^2+2.80.1+1^2=6400+160+1=6561

91^2=(90+1)^2=90^2+2.90.1+1^2=8100+180+1=8281

Answer 3

b)

19.21=(20−1).(20+1)=20^2−1^2=400−1=399

29.31=(30−1).(30+1)=30^2−1^2=900−1=899

39.41=(40−1).(40+1)=40^2−1^2=1600−1=1599

Answer 4

c)

29^2−8^2=(29−8).(29+8)=777

56^2 - 46^2 = ( 56 - 46).(56+46) = 1020

67^2 - 56^2 = ( 67 - 56).(67 + 56) = 1353