Question

4+7+10+...+91
6+11+16+21+......+101

Answer 1

4 + 7 + 10 +...+ 31 =

= 3×1+1 + 3×2+1 + 3×3+1 +...+ 3×10 + 1 =

= 3×(1 + 2 + 3 +...+ 10) + 1 + 1 + 1 +...+ 1 de 10 ori

= 3 × 10 × 11 : 2 + 1 × 10 =

= 3 × 5 × 11 + 10 =

= 165 + 10 =

= 175


6+11+16+21+......+101 =
5×1+1 + 5×2+1 +5×3+1 +5×4+1 +...+ 5×20+1 =
= 5×(1+2+3+4+...+ 20) + 1+1+1+...+1 de 20 ori
= 5 × 20 × 21 : 2 + 1×20 =
= 5 × 10 × 21 + 20 =
= 50×21 + 20 =
= 1050 + 20=
= 1 070