a: \(\left(2x+1\right)\left(2x+3\right)\left(x+1\right)^2-18\)
\(=\left[\left(2x+2\right)^2-1\right]\left(x+1\right)^2-18\)
\(=4\left(x+1\right)^4-\left(x+1\right)^2-18\)
\(=4\left(x+1\right)^4-9\left(x+1\right)^2+8\left(x+1\right)^2-18\)
\(=\left(x+1\right)^2\left[4\left(x+1\right)^2-9\right]+2\left[4\left(x+1\right)^2-9\right]\)
\(=\left[\left(2x+2\right)^2-9\right]\left[\left(x+1\right)^2+2\right]\)
\(=\left(2x+5\right)\left(2x-1\right)\left(x^2+2x+3\right)\)
b: \(\left(x^2+4x+3\right)\left(x^2+12x+35\right)+15\)
\(=\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15\)
\(=\left(x^2+8x+7\right)\left(x^2+8x+15\right)+15\)
\(=\left(x^2+8x\right)^2+22\left(x^2+8x\right)+105+15\)
\(=\left(x^2+8x\right)^2+22\left(x^2+8x\right)+120\)
\(=\left(x^2+8x+10\right)\left(x^2+8x+12\right)\)
\(=\left(x^2+8x+10\right)\left(x+2\right)\left(x+6\right)\)
c: \(\left(x-3\right)\left(x-5\right)\left(x-6\right)\left(x-10\right)-24x^2\)
\(=\left(x^2-13x+30\right)\left(x^2-11x+30\right)-24x^2\)
\(=\left(x^2+30\right)^2-24x\left(x^2+30\right)+143x^2-24x^2\)
\(=\left(x^2+30\right)^2-24x\left(x^2+30\right)+119x^2\)
\(=\left(x^2-17x+30\right)\left(x^2-7x+30\right)\)
\(=\left(x-2\right)\left(x-15\right)\left(x^2-7x+30\right)\)