The rate of the tax imposed by and computed under this chapter is:

(1) Six and six-tenths percent of the first $1 million of taxable income, or fraction thereof; and

(2) Seven and six-tenths percent of any amount of taxable income in excess of $1 million. [1975 c.368 § 2; 1983 c.162 § 5; 1987 c.293 § 34a; 2009 c.745 5,7,9; 2013 s.s. c.5 § 1]