Решение:
а) \( \int \frac{3}{7 \sin^2 x} dx \)
= \( \frac{3}{7} \int \frac{1}{\sin^2 x} dx \)
= \( \frac{3}{7} (-\cot x) + C \)
= \( -\frac{3}{7} \cot x + C \)
б) \( \int (4x^2 + \frac{3}{\cos^2 x}) dx \)
= \( \int 4x^2 dx + \int \frac{3}{\cos^2 x} dx \)
= \( 4 \int x^2 dx + 3 \int \frac{1}{\cos^2 x} dx \)
= \( 4 \cdot \frac{x^3}{3} + 3 \tan x + C \)
= \( \frac{4}{3}x^3 + 3 \tan x + C \)
в) \( \int (2 \cos x - 3) dx \)
= \( 2 \int \cos x dx - 3 \int dx \)
= \( 2 \sin x - 3x + C \)