Question

Caluleaza : 4+8+12+...+400 =

Answer 1

$\frac{4*(1+2+...+100)}{1} = \frac{4*(100*101)}{2} =4*5050=20200$

Answer 2

$\displaystyle 4+8+12+...+400=4(1+2+3+...+100)=4 \cdot \frac{100(100+1)}{2} = \\ \\ =4 \cdot \frac{100 \cdot 101}{2} =4 \cdot \frac{10100}{2} =4 \cdot 5050=20200$