$$\frac { 10 \times 10 \times ( 10 + 1 ) + - 9 \times 10 } { 2 }$$
$505$
$$\frac{100\left(10+1\right)-90}{2}$$
$$\frac{100\times 11-90}{2}$$
$$\frac{1100-90}{2}$$
$$\frac{1010}{2}$$
$$505$$
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$5\times 101$
