a: \(=\left(x^2+8x+7\right)\left(x^2+8x+15\right)+16\)
\(=\left(x^2+8x\right)^2+22\left(x^2+8x\right)+121\)
\(=\left(x^2+8x+11\right)^2\)
b: \(=\left(x^2-5xy+4y^2\right)\left(x^2-5xy+6y^2\right)+y^4\)
\(=\left(x^2-5xy\right)^2+10y^2\left(x^2-5xy\right)+25y^4\)
\(=\left(x^2-5xy+5y^2\right)^2\)