The first three terms, in descending powers of x, in the expansion of (3x²-a)ⁿ (1+2/x²)² can be written as 729x¹² - 972x¹⁰ + bx⁸, where a, b and n are constants. Find the values of a, b and n.
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [B2] n = 6 stated; nfww (B1 for first term: (3x²)ⁿ or (3x²)⁶ soi)
- [B3] -6(3x²)⁵a + 2(729x¹⁰) = -972x¹⁰ oe (B2 for one correct equation)
- [B2] a = 1/3
- [B2] b = -108
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