Consider the indefinite integral \int x \sqrt{1+x^{2}} d x Which of the following u-substitutions should we make to transform this integral \text { into } \frac{1}{2} \int u^{1 / 2}

d u \text { ? } u=\frac{1}{2} x u=1+x^{2} u=\frac{1}{2}+\frac{1}{2} x^{2} u=\sqrt{1+x^{2}} u=x^{2}