Find the sum of the first 90 positive odd integers

how do I work this out?

The nth odd integer is 2n-1.

1 + 3 + 5 + ... + 179 = x
179 + 177 + ... + 1 = x
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180 + 180 + ... + 180 = 2x

90·180 = 2x
x = 8100