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Let us write ,  x = 5 - (5-x)

Hence given integration is

Let (5-x)  = u  and dx = -du

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Second integration as given below is not a proper definite integral .

...............................(1)

In a definite integral     , function f(x) should be finite between the limits a and b .

If the function is infinite at any point between a and b , then integration can not be performed .

In the given integraion as given in eqn.(1) , integrand becomes infinite at x = 1 .

Hence integration given eqn.(1) can not be performed,