Question

The first 3 terms in the expansion of (2+3x)^5 in ascending powers of x is 32 + 240x + 720x2;

i) Find the the value of the constant of a for which there is no term in xin the expansion of (1+ax)(2+3x)^5

Answer 1

*
for which there is no term in x^2 *

Answer 2

sry the first 3 terms are 32+240x+720x^2

Answer 3

group the x^2 terms and set equal to 0
720 +240a = 0
a = -3

Answer 4

how do u group them

Answer 5

(1+ax)(32+240x+720x^2) = 720x^2+ 240ax^2 +240x +32ax +32 +720ax^3
the x^2 terms are --> 720x^2+ 240ax^2

Answer 6

why do we group the x^2 terms and why do we set equal to 0?

Answer 7

For there to not be an x^2 term the coefficient must be 0
when adding like terms you add the coefficients of all the x^2 terms

Answer 8

k ryt thanks