0606 P21 - Jun 2020 - Q8 - 7 marks
8135
(a) Expand \((2-x)^5\), simplifying each coefficient.
(b) Hence solve
\(\frac{e^{(2-x)^5}\times e^{80x}}{e^{10x^4+32}}=e^{-x^5}.\)
Solution
Answer: \(32-80x+80x^2-40x^3+10x^4-x^5\); \(x=0\) or \(x=2\).
(a) Use the binomial expansion:
\((2-x)^5 =2^5+5(2^4)(-x)+10(2^3)(-x)^2+10(2^2)(-x)^3+5(2)(-x)^4+(-x)^5.\)
So
\((2-x)^5=32-80x+80x^2-40x^3+10x^4-x^5.\)
(b) Combine the powers of \(e\) on the left:
\(\frac{e^{(2-x)^5}\times e^{80x}}{e^{10x^4+32}} =e^{(2-x)^5+80x-(10x^4+32)}.\)
Using the expansion from part (a), the exponent is
\(32-80x+80x^2-40x^3+10x^4-x^5+80x-10x^4-32.\)
This simplifies to
\(80x^2-40x^3-x^5.\)
The equation becomes
\(e^{80x^2-40x^3-x^5}=e^{-x^5}.\)
Therefore
\(80x^2-40x^3-x^5=-x^5.\)
So
\(80x^2-40x^3=0.\)
Factorising,
\(40x^2(2-x)=0.\)
Hence
\(x=0\quad\text{or}\quad x=2.\)