Question

if 2a=3b-5 find 8a^3-27b^3+90ab+125

Answer 1

Give:

$2a=3b-5$

And the given expression is

$8a^3-27b^3+90ab+125$

Which can also be written as

$(2a)^3 -27b^3 +45b(2a)+125$

Substituting the vale of 2a

$(3b-5)^3-27b^3+45b(3b-5)+125 \\ = 27b^3 -135b^2+225b-125-27b^3 +135b^2 -225b+125 \\ =0$