Question

calculati:a)1+2+3....+56+57+58;b)1+2+3+....+35+36+37;c)2+4+6+.....+86+88+90;d)1+5+10+15+...+65+70

Answer 1

a) 1+2+3....+56+57+58=

= (58x59):2 = 1171

b) 1+2+3+....+35+36+37=

= (37x38):2=

= 703

c) 2+4+6+...+86+88+90= 90 este de forma 2n => n=45

= 45x46=2070

d)1+5+10+15+...+65+70=

= 1+5(1+2+3+...+13+14)=

= 1+5(14x15):2=

=1+5x105=526