Welcome to the Dell Community [NAME]  1x8GB  +  1x4GB for a total of 12GB should work. The 4GB stick and the first 4GB of the 8GB stick will run in "Dual Channel/Interleave" mode. The remaining 4GB of the 8GB stick will run in "Single Channel" mode. Yes this will increase the total memory, but it can cause a loss in system performance as the memory will be switching between Single and Dual channel. It is best to install "Matching Pairs" for the best performance. Best regards, U2
Hi, I just came across post on this forum for RAM upgrade where two cards were used i.e. 8gb + 8gb I have N5110 i5, 4gb, 1 TB HDD which now I am upgrading it to 1 TB ssd and want to add 8gb memory card. I dont want to throw away existing 4gb memory card and just want to add 8 gb card on top of that to make it 12gb. will it work? or only 4gb card will work with 4gb? 
Inspiron N5110 RAM upgrade 4gb + 8gb