To evaluate \int \frac{3 x}{\sqrt{x^{2}-49}} d x which of the following substitution should be used? \text { a. } x=49 \sec \theta \text { b. } x=7 \sec \theta \text { c. } x=7 \tan \theta \text { d. none of these. } \text { e. } x=7 \sin \theta

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