Răspunsuri

2014-07-04T17:00:47+03:00
B) se mai poate scrie ca x+2x+3x+....20x+y+2y+3y+....20y= x(1+2+3+...+20)+y(1+2+3+...+20)= (1+2+....+20)(x+y)=20*21/2(x+y)=210(x+y)
S- a folosit formula
1+2+....+20=20*(20+1)/2
a)  2x+2y=2(x+y)=2*3=6
     15x+15y=15(x+y)=15*3=45
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2014-07-04T17:02:50+03:00
1)
x + y = 3

a) calculati:
2x + 2y = 2(x + y) = 2 * 3 = 6
15x + 15y = 15(x + y) = 15 * 3 = 45

b) Calculati suma urmatoare:
(x + y) + (2x + 2y) + (3x + 3y)+................ + (20x + 20y) =
= (x + y) (1 + 2 + 3 + ........20 ) = 
= (x + y ) (20 * 21 / 2 ) =
= (x + y) (10 * 21) =
= 3 * 210 = 630