I am having problem when it come to integration

∫(0 to 1) t^3 x sqrt(t^2 +1) dt

I try to use u substitution saying that

u= t^2 +1

du = 2t dt

then i don't know what to do because when I plug it in

it is like

∫(0 to 1) (1/2t) t^3 x sqrt(u) du

∫(0 to 1) (1/2) t^2 x sqrt(u) du

I dont' know what to do with t^2

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And same with this problem

∫(0 to 1) t sin(3+3t) dt

so I try to use u substitution like

u= 3+3t

du= 3 dt

then I plug it in

∫(0 to 1) (1/3) t sin(u) du

and I got same problem again. I don't know what to do with t

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Am i using wrong way of integration? I am not really sure what am i doing wrong? I just can't continue