x2-81 = (x-9)(x+9)
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4x^(2) - 81 First remember the two numbers are both squared numbers. 4 = 2^(2) & 81 = 9^(2) Hence (2x)^(2) - 9^(2) Next remember two squared terms with a negative(-) between them , will factor. Hence (2x - 9)(2x + 9) Note the two different signs. NB Two squared terms with a positive (+) between them does NOT factor. .
well, the square root of 81 is 9, and seven squared is 49, so it wud be 40
81 x 81 = 6561
-9 squared = 81(-9)^2 = (-9) * (-9) = 81
812 = 81 x 81 = 6,561