（1）5c平方分之6ab*3b分之10c （2）x平方+4x+4分之x平方-4y平方*3x平方+6xy分之x+2

（1）6ab/5c^2*10c/3b
=4a/c
(2)(x^2-4y^2)/(x^2+4x+4)*(x+2)/(3x^2+6xy)
=(x+2y)(x-2y)/(x+2)^2*(x+2)/3x(x+2y)
=(x-2y)/3x(x+2)
(3)(x^2+1)/(x-6)*(x^2-36)/(x^3+x)
=(x^2+1)/(x-6)*(x+6)(x-6)/x(x^2+1)
=(x+6)/x
(4)(y^2-x^2)/4xy*(5x-4y)/(x+y)
=(y-x)(5x-4y)/4xy