how to integrate using u substitution

Understand u-substitution, 6 examples 20 examples on "How to choose the u":    • u substitution: how to choose the "u"   integral of sec(2x)tan(2x), 0:30 integral of sec^2x*tan^2x, 3:57 integral of sin(x)/(1+cos(x)), 8:07 integral of sin(x)/(1+cos^2x), 16:53 integral of x*sqrt(x^2-1), 23:22 integral of x*sqrt(x-1), 27:00