Question

calculati:a) 2005-754+107-238;b) 1+3+5-. ...+19

Answer 1

a)

2005 - 754 + 107 - 238 = 1 120

= 1 251 + 107 - 238 =

= 1358 - 238 =

= 1 120

______________________

b)

1 + 3 + 5 + ....... + 19 = 100

= ( 1 + 19 ) + ( 3 + 17 ) + ( 5 + 15 ) + ( 7 + 13 ) + ( 9 + 11 ) =

= 20 + 20 + 20 + 20 + 20 =

= 100  

sau aplic formula sumei lui Gauss:

= [ ( 19 - 1 ) : 2 + 1 ] termeni × ( 1 + 19 ) : 2 =

= 10 × 20 : 2 =

= 200 : 2 =

= 100