Answer: Distributive property
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In general the distribute property is this
a*(b+c) = a*b + a*c
or
(b+c)*a = b*a + c*a
the basic idea is that we multiply the outer term 'a' with each term inside (those terms being b and c).
In your problem, the outer term is 8 and the inner terms are 9 and d
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Another Example:
(2+3)*4 = 2*4+3*4 = 8+12 = 20
note how
(2+3)*4 = 5*4 = 20