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Answer:
b. (r-9) (r+9)
Step-by-step explanation:
In (r-9) (r+9) using foil method
F = r²
O = +9r
I = -9r
L = -81
so:
r²-81
Explaination:
r times r is equal to r²
r times positive 9 is equal to +9r
r times negative 9 is equal to -9r
+9 multiply by -9 is equal to -9 (negative by negative = positive, positive by negative = negative, negative by negative = positive)
+9r and -9r will cancel each other so 0.