Q4.
y=\frac{1}{x^2}
becomes
\frac{1}{\sqrt{y}}
Thus,
\int^{2}_{1}\frac{1}{\sqrt{y}} \, dy
self explanatory from there on;
Q6.
A1,
\int^{2}_{1}x^2 \, dx
=
[\frac{x^3}{3}]^{2}_{1}=\frac{7}{3}
A2,
x^2=y
=
x=\sqrt{y}
=
\int^{4}_{1}{\sqrt{y} }\, dy
=
[\frac{2(y^{\frac{5}{3}})}{3}]^4_1
=\frac{14}{3}...