Question

2+4+6+8+....+198


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Answer 1

2(1+2+3+...+99)=2*99*(99+1)/2=9900

Answer 2

$\displaystyle 2+4+6+8+...+198=2(1+2+3+...+99)=2 \times \frac{99(99+1)}{2} = \\ \\ =2 \times \frac{99 \times 100}{2} =\not 2 \times \frac{9900}{\not 2} =9900$