Explicație pas cu pas:
A)
7+8+9+..................+54=
=(54+7)×[(54-7):1+1]:2
=61×(43:1+1):2
=61×(43+1):2
=61×44:2
=61×22
=1342
B)
15+16+17+.............+80=
=(80+15)×[(80-15) :1+1] :2
=95×(65:1+1):2
=95×(65+1):2
=95×66:2
=95×33
=3135
C)
31+32+33+..........+100=
=(100+31)×[(100-31):1+1]:2
=131×(69:1+1):2
=131×(69+1):2
=131×70:2
=131×35
=4585
Sper că te-am ajutat! Spor!