In this paper we investigate Diophantine equations of the form , , where m=3 or m=4 and G is a specific homogeneous quintic form. First, we prove that if F(x,y,z)=x2+y2+az2+bxy+cyz+dxz∈Z[x,y,z] and (b−2,4a−d2,d)≠(0,0,0), then for all n∈Z鈭杮0} the Diophantine equation t2=nxyzF(x,y,z) has a solution in polynomials x, y, z, t with integer coefficients, with no polynomial common factor of positive degree. In case a=d=0, b=2 we prove that there are infinitely many primitive integer solutions of the Diophantine equation under consideration. As an application of our result we prove that for each n∈Q鈭杮0} the Diophantine equation
has a solution in co-prime (non-homogeneous) polynomials in two variables with integer coefficients. We also present a method which sometimes allows us to prove the existence of primitive integer solutions of more general quintic Diophantine equation of the form , where a,b,c,d∈Z. In particular, we prove that for each m,n∈Z鈭杮0}, the Diophantine equation
has a solution in polynomials which are co-prime over Z[t]. Moreover, we show how a modification of the presented method can be used in order to prove that for each n∈Q鈭杮0}, the Diophantine equation
has a solution in polynomials which are co-prime over Z[t].