Question

Làm giúp nha 

1.Tính nhanh các tổng sau

a) 1+2+3+4+....+n

b)1+3+5+7+.....+(2n-1)

c)2+4+6+8+.......+2n

d)1+6+11+16+.......+46+51

Answer 1

a)1+2+3+...+n

=[(n-1):1+1].(n+1):2

=n.( n+1)/2

b) {[(2n-1)-1]:2+1}. [(2n-1)+1]:2

=n.n=n²

Answer 2

a) 1+2+3+...+n

= [(n-1):1+1].(n+1):2

= n.( n+1)/2

b) {[(2n-1)-1]:2+1}. [(2n-1)+1]:2

= n.n = n²

Answer 3

a) 1 + 2 + 3 +...+ n

= [( n - 1 ) : 1 + 1 ] . ( n + 1 ) : 2

= n . ( n + 1 )/2

b)  1 + 3 + 5 + 7 +...+ ( 2n - 1 )

= {[( 2 n - 1 ) - 1 ] : 2 + 1 } . [( 2 n - 1 ) + 1 ] : 2

= n . n = n²

Hk tốt