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Simplifying
4r2 + -9 = 81
Reorder the terms:
-9 + 4r2 = 81
Solving
-9 + 4r2 = 81
Solving for variable 'r'.
Move all terms containing r to the left, all other terms to the right.
Add '9' to each side of the equation.
-9 + 9 + 4r2 = 81 + 9
Combine like terms: -9 + 9 = 0
0 + 4r2 = 81 + 9
4r2 = 81 + 9
Combine like terms: 81 + 9 = 90
4r2 = 90
Divide each side by '4'.
r2 = 22.5
Simplifying
r2 = 22.5
Take the square root of each side:
r = {-4.74341649, 4.74341649}
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