Answer

(1)
y^'
= {x²(3x+1)}^'
= (x²)^'(3x+1) + x²(3x+1)^'
= 2x(3x+1) + x²Ľ3
=9x² + 2x

(2)
y^'
= {e^xcos x}^'
= (e^x)^'cos x + e^x(cos x)^'
= e^xcos x - e^xsin x

(3)
y^'
= (sin x cos x)^'
=(sin x)^'cos x + sin x(cos x)^'
=cos²x - sin²x
=cos 2x