Algebramedium · Past Paper
Factorize a^4 - 5a^2 + 4.
A(a-1)(a+1)(a-2)(a+2)
B(a^2-4)(a^2-1)
C(a-1)^2(a-2)^2
D(a+1)^2(a+2)^2
✓ Correct Answer: A — (a-1)(a+1)(a-2)(a+2)
Treat a^2 as u: u^2-5u+4 = (u-4)(u-1) = (a^2-4)(a^2-1) = (a-2)(a+2)(a-1)(a+1).
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