# binominal expansion

• February 10th 2009, 02:32 AM
thereddemon
binominal expansion
4. When (1-2x)^p is expanded, the coefficient of x^2 is 40. Given that p>0, use this information to find:
1. the value of te constant p.
2. the coefficient of x.
3. The coefficient of x^3.
Explain it to me please!!! Thank you
• February 10th 2009, 04:34 AM
Plato
Quote:

Originally Posted by thereddemon
4. When (1-2x)^p is expanded, the coefficient of x^2 is 40. Given that p>0, use this information to find:
1. the value of te constant p.
2. the coefficient of x.
3. The coefficient of x^3.
Explain it to me please!!! Thank you

$\left( {1 - 2x} \right)^p = \sum\limits_{k = 0}^p {{p \choose k}\left( { - 2x} \right)^k }$ so ${p \choose 2} 40x^2 = \left( { - 2x} \right)^2$.
Solve for $p$.