Change in order of integration in an integral

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I do not understand the change of order of integration ($dtdx$ to $dxdt$, RHS second line). Why the $\frac{1}{2}$ term? Why do we integrate $x$ from $t$ to $1$?

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You are right. There shouldn't be a $1/2$ there. It should appear after the change $x^2\to x$.

The change in the integration limits is correct. To see why, draw the region of integration of both $\int_0^1\int_0^x f(x,t)\, dt\, dx$ and $\int_0^1\int_t^1 f(x,t)\, dx\, dt$.