How many of the following equations are partial differential equations?
a) u^{}2_{}x(x, y)+2u_{yy}(x, y)=cos(2x+5y)
b)3u_{}x(x, y)+2u_{yy}(x, y)=0
c)(iii) u_{xx}(x, t)= -u_{tt}(t, x)-u_{t}(t, x)
d)3u_{xx}(x, y)+2u_{yy}(y, x)=3u_{}x(x, y)
e)u_{xx}(x, y)+3u_{yy}(x+\pi ,y)=-u_{}x(x, y)+u_{}y(x, y-\pi )
f)ey^{}2u_{}x(x, y)+x^{}4u_{}y(x, y)=tan(x)
g)u^{}2_{}x(x, y)+2u_{yy}(x, y)=cos(2x+5y)