Răspuns:
ex 14
Grupam termenii astfel...
a) (574+26) + (875+25) =600+900=1500;
b) (2036+64) + (871+329) + 50 = 2100+1200+50=3350;
c) (2031+689) + (3698+302) = 2720+4000=6720;
d) (324+76) + (109+1)+100+16=400+110+116=626;
ex 15
a) 1 + 2 + 3 + ..........+ 80 = 80 * (80 + 1)/2 = 40 * 81 = 3240
b) 2 + 4 + 6 + .......+ 100 = 2 * 50 * (50 + 1)/2 = 2 * 51 * 25 = 2550
c) 1 + 3 + 5 + ........ + 99 = (99 + 1) * [(99 - 1)/2 + 1] : 2 = 100 * [49 + 1] : 2 =
100 * 25 = 2500
d) 3 + 7 + 11 + 15 + ...... + 43 = (43 + 3) * [(43 - 3) : 4 + 1] : 2 = 46 * [40 : 4 + 1] : 2 = 46 * 11 : 2 = 23 * 11 = 253
