Question

Calcula. ti 11+22+33+44+........7700 21+42+63+84+.....2100. Vreau detaliat sa anteleg

Answer 1

$\displaystyle a).11+22+33+44+...+7700=11(1+2+3+...+700)= \\ \\ =11 \times \frac{700(700+1)}{2} =11 \times \frac{700 \times 701}{2} =11 \times \frac{490700}{2} = \\ \\ =11 \times 245350=2698850 \\ \\ b).21+42+63+84+...+2100=21(1+2+3+...+100)= \\ \\ =21 \times \frac{100(100+1)}{2} =21 \times \frac{100 \times 101}{y} =21 \times \frac{10100}{2} = \\ \\ =21 \times 5050=106050$