125500 漲4次利息
第k年還款x元，會漲(4-k)次利息 (k=1,2,3,4)
125500 · (1.03)⁴ = x · (1.03)³ + x · (1.03)² + x · (1.03) + x
= x · [ 1 + (1.03) + (1.03)² + (1.03)³ ]
= x · [(1.03)⁴-1]/[1.03-1]
125500 · 1.1255 = x · (0.1255)/(0.03)
x = (125500 · 1.1255 · 0.03)/(0.1255)
= 10⁶ · 1.1255 · 0.03
= 3.3765 · 10⁴
= 33765
≈ 33800
