Below, you will find the mathematical proof that 10 equals 9.99999?. But is that possible or there is something wrong about it? Can you find the error?

x = 9.999999...
10x = 99.999999...
10x - x = 90
9x = 90
x = 10

Spot 5 differences in two Christmas tree below:

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131 517 192 123 ?

Can you arrange four 9's and use at most 2 math symbols, to make the total 100?