Question

ex 1 10+20+30+...+800=
ex 2 13+26+39+...+715=
ex 3 21+42+63+...+1890=
vă rog ajutați-mă ​

Answer 1

a) 10+20+30+....800=  

10(1+2+3+....80)=

10(80x81):2=  

10*6480:2=

64800:2=32400

b)13+26+39+...715=

13(1+2+3+...55)=

13*(55x56):2=

13*3080:2=

13*1540=20020

c) 21+42+63+.....+1890 =

21+42+63+.....+1890=21×(1+2+3+...+90)=21[(90×91):2]=85995

(1+2+3+...+90)=

21×[(90×91):2]=85995

Trevorr