Alan

Yes, the sequence is geometric.

Mathcad gives the following:

.

At the beginning of the 1st year it is worth  1250

At the beginning of the 2nd year it is worth  1250*0.9

At the beginning of the 3rd year it is worth  1250*0.9*0.8  → 1250*(0.9²⁾

...

At the beginning of the n'th year it is worth  1250*(0.9^n-1)

Try taking it from here.

The sequence is:  9  11   13   15 ....etc.

a.  If the n'th term is r_n then  r_n = r_n-1 + 2

b.  The sequence can be written as 9  9+2   9+2*2   ...   9+(n-1)*2 ... etc.  so, explicitly: r_n = 9 + 2(n-1)  or r_n = 7+2n

c.  You should be able to wotk out r₁₂ using the result in b.

Force is mass times acceleration, so just multiply the numbers together.  With the specified units for mass and acceleration the force will be in Newtons.

81^0.25 = 3

2^-1 = 1/2

Can you take it from here?

Let a, b and c be the side lengths with c being the longest.  

If c^2 > a^2 + b^2 the triangle is obtuse.

If c^2 < a^2 + b^2 the triangle is acute.

The "l" in "ln" is a lower case L not an upper case "i"!!  Unfortunately, The two look identical in many fonts!

You are correct.  The ln means the natural logarithm; that is, logarithm to the base e.  However you could use logarithm to the base 10 if you prefer (as long as you use the same base for both the 925 and the 3).

(You should find t is just over 6.)

Ok, so it's the second of my replies above that is the one you want.