The CPU has nothing to do with compatibility. The RAM from the 7720 is DDR3 and the RAM required for the 5759 is DDR3L. You can put DDR3L into a DDR3 SODIMM slot but you cannot put DDR3 RAM into a DDR3L slot. The DDR3 RAM requires higher voltage than the DDR3L slot supplies. This web page explains the differences.
Is Intel Core i7 RAM 2.3 Ghz compatible with the Intel Core i3 RAM 2.3 Ghz in the Inspiron 5759?  I would like to increase RAM in the 5759 from 4K to 8K by using the i7 RAM from an older Inspiron 17R SE 7720.
Adding i7 RAM to Inspiron 5759 i3 RAM?