a) 1+2+3+...+50
[tex]=\frac{50(50+1)}{2} =\frac{50*51}{2} =\frac{2550}{2}=1275[/tex]
b) 2+4+6+...+20
[tex]=2(1+2+3+...+10)\\\\=2[\frac{10(10+1)}{2} ]\\\\=2(\frac{10*11}{2} \\\\=2(\frac{110}{2})\\\\=2* 55\\\\=110[/tex]
a)
1+2+3+4+....+50 =
50×(50+1):2 =
50×51:2 =
25×51 =
1275
b)
2+4+6+8+.....+20 =
2×(1+2+3+4+......+10) =
2×[10(10+1):2] =
2×(10×11:2) =
2×5×11 =
110
