In high numbers, we know that the Laplace equation U= 1/ radical sign (x 2+y 2+z 2) satisfies that the sum of the second-order partial derivatives of U to x, y and z is equal to 0 (that is, the formula satisfied by U in the title), so the functional relationship F should be 1/r +C (the previous sign may be) So f(t)= 1/t+C, or-1/t+C. Then from the known f( 1)=0, f? (1)= 1, f(t) is the second relation, and C= 1. To sum up, f(t)=- 1/t+ 1.