Đáp án:
a) 121² - 21²= (121 - 21 )( 121 + 21)= 100 × 142= 14200
b) 67² -33² = (67+ 33) ( 67 - 33) = 100 × 34 = 3400
c) 101² = (100 +1)² = 100² + 2·100 +1 = 10000 + 200 +1 = 10201
d) 39× 41= (40 -1) ( 40+1) = 40² - 1² = 1600 - 1= 1599
e) 58² + 84 · 58 + 42² = 58² + 2· 42·58 + 42² = (58 + 42)² = 10000
f) 99² + 99 · 2 +1 = (99 +1)² = 100²= 10000