I'm need some assistance with intermediate information for a problem I'm attempting to solve. For

$\displaystyle f(x,y)=\left\{ \begin{array}{cc} K(x^{2}+y^{2}) & 20\leq x\leq30,20\leq y\leq30\\0 & \textrm{otherwise}\end{array}\right.$

I'm attempting to solve for E(X) . As per the definition,

$\displaystyle E(X)=\int_{20}^{30}x\cdot f_{x}(x)dx$

Where my confusion lies is, based on my calculations,

$\displaystyle f_{x}=2Kx$

But, according to my solutions manual (corroborated by other sources),

$\displaystyle f_{x}=10Kx^{2}+.05$

I don't understand how they obtained these values. What am I missing here?