中4 maths 2條

13)4n(n+1) = 8(4 + n + n+1)n² + n = 2(5 + 2n)n² - 3n - 10 = 0(n - 5)(n + 2) = 0n = 5 或 n = - 2(捨)n 和 n+1 是 5 和 6 , (B)
14)x + (14 - x) + √[x² + (14 - x)²] = 24√[x² + (14 - x)²] = 10x² + (14 - x)² = 100x² + x² - 28x + 196 = 1002x² - 28x + 96 = 0x² - 14x + 48 = 014x - x² = 48x(14 - x) = 48(1/2)(x)(14 - x) = 24△面積 = 24cm² , (C)