📝 题目
5.5.7 设 $F\left( x\right) = {\int }_{0}^{2\pi }{\mathrm{e}}^{x\cos \theta }\cos \left( {x\sin \theta }\right) \mathrm{d}\theta$ ,求证: $F\left( x\right) \equiv {2\pi }$ .
5.5.7 设 $F\left( x\right) = {\int }_{0}^{2\pi }{\mathrm{e}}^{x\cos \theta }\cos \left( {x\sin \theta }\right) \mathrm{d}\theta$ ,求证: $F\left( x\right) \equiv {2\pi }$ .