If $$a^{3} - b^{3}$$ = 91 and a - b = 1, then what is the value of ab?
Given :  $$a^{3} - b^{3}$$ = 91 -----------(i)
Also, $$a-b=1$$ -----------(ii)
Cubing both sides, we get :
=> $$(a-b)^3=(1)^3$$
=> $$a^3-b^3-3ab(a-b)=1$$
Substituting values from equation (i) and (ii)
=> $$91-3ab(1)=1$$
=> $$3ab=91-1=90$$
=> $$ab=\frac{90}{3}=30$$
=> Ans - (D)
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