60. Find the general form of numbers whose squares end in the digit 9.
Instruction. Numbers whose squares end in the digit 9 have the digits 3 or 7 in the units place, i.e., they have the form:

$10a+7, 10a+3; 10a+7=5(2a+1)+2, 10a+3=5(2a+1)-2$, so these numbers have the form: $5(2a+1) \pm 2$ or $10b \pm 3$.