A negative exponent means reciprocal. For example, 2^-1 means 1/2; 3^-4 means 1/3^4=1/81. In general, then, x^-y means 1/x^y. You can have fractional negative exponents, too: 8^-1/3 means 1/cuberoot(8)=1/2. You can include positive and negative exponents in one expression: a^x times b^-y means a^x/b^y.