Square root method is best method to solve the given equation.
Explanation:
The given equation x²-81=0 can be best solved by the square root method. The square root method is best applicable when both the terms are perfect squares. In the equation, x²-81=0, all the terms are perfect squares.
After segregation of terms, a simple square root of the equation would yield the desired result.
x²=81
∴ x= ±9
Since the equation is a polynomial of 2 degrees, the equation would have 2 solutions. Hence the value of x can be either +9 or -9.