Then there is F(x? ) -F(0), where F(0) is a constant of F'(0)=0,

Let t=x? , so (F(x? )-F(0))'

=F'(x？ )=t'f(t)

=2xf(t), and t=x? Substitute,

That is, the derivative rule of composite function,

So the variable limit integral = 2x √ (1+x 4).

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