# C is invertible
C <- matrix(c(1.9,1.4,1.4,1.4,1.9,1.4,1.4,1.4,2.0),3,3)
# D is NOT invertible
D <- matrix(c(.289,.109,0.0,.109,1.46,0.0,0,0,0),3,3)

# E is invertible
E <- matrix(c(4.0,3.0,2.0,3.0,5.0,1.0,2.0,1.0,6.0),3,3)
# F is NOT invertible
F <- matrix(c(0,0,0,0,3,1.5,0,1.5,3),3,3)

A <- solve(C) %*% D
B <- solve(E) %*% F
I <- diag(3)

# How to find solve(solve(A) + solve(B))?
#
# If A and B were equal, then the answer is
# solve(2*C) %*% D