Question

Calculati:
a) 1+2+3+...+50
b) 2+4+6+...+20​

Answer 1

a) 1+2+3+...+50

$=\frac{50(50+1)}{2} =\frac{50*51}{2} =\frac{2550}{2}=1275$

b) 2+4+6+...+20

$=2(1+2+3+...+10)\\\\=2[\frac{10(10+1)}{2} ]\\\\=2(\frac{10*11}{2} \\\\=2(\frac{110}{2})\\\\=2* 55\\\\=110$

Answer 2

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