A big cake 1 knife cuts 2 pieces, 2 knives cuts 4 pieces, and 3 knives cuts () pieces.

Answer: 7

Think of the pie as an airplane. Its law is: the essence is the principle of dividing a circle by a straight line. A straight line divides a circle into two parts at most, and the two parts can be divided into four parts at most ... If you want to be the most, that is to say, every redundant straight line intersects with the original straight line.

One knife: 1+ 1=2

Two knives: 1+ 1+2=4.

Three knives: 1+ 1+2+3=7.