大二MATH的几个题请大家帮忙.很着急.

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大二MATH的几个题请大家帮忙.很着急.
mathematics of investment的几个题
#1
A person's saving earn an effective rate of i=0.12 on which 45% income tax is paid.If the inflation rate is 10% per year,what is the annual after--tax real rate of return?
#2
A customer is offered an investment where is calculated according to the following force of interest:
St= 0.02t o
前3题知道怎么做了.......帮我看下第4题吧..
#4
(a)At an effective annual interest rate of 12%,calculate the number of years(including fractions) it will take for an investment of 1000 to accumulate to 3000.
(b)Repeat part (a) using assumption that for fractions of a year,simple interest is applied.
(c)Repeat part (a) using an effective monthly interest rate of 1%.
(d)Suppose that an investment of 1000 accumulated to 3000 in exactly 10 years at effective annual rate of interest i.Calculate i.
(e)Repeat part (d) using an effective monthly rate of interest j.Calculate j.

（a）
x
1000 × (1+12%) = 3000
x
(1+12%) =3000÷1000
3
X=log =9.694
1.12
(b) 看不懂
(c) x
1000 × （1+1%） = 3000
x
（1+1%） = 3000÷1000
3
X=log =110.4
1.01
10
(d) 1000×(1+i) =3000
i=11.6%
10x12
(e) 1000x(1+j) =3000
1+j=√(120&3)
j=0.91%

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