S5=10 means the 5th number is 10.
So the arithmetic sequence is s1, s2,s3, s4, 10
Answer:
Part 1) 
Part 2) 
Part 3)
and 
Step-by-step explanation:
see the attached figure with letters to better understand the problem
step 1
Find the side b
we know that
In the right triangle ABC
The function sine of angle B is equal to divide the opposite side angle B (AC) by the hypotenuse (AB)

we have



substitute

solve for b


step 2
Find the side a
we know that
In the right triangle ABC
The function cosine of angle B is equal to divide the adjacent side angle B (BC) by the hypotenuse (AB)

we have



substitute

solve for a


step 3
Find the measure of angle A
we know that
In the right triangle ABC
----> is a right angle

∠A+∠B=90° ------> by complementary angles
substitute the given value



Answer:
Answer: (2a - 5 + b)5
Answer: 10 x (a - 2.5 + 0.5b)
Answer: (-2a + 5 - b) ⋅ (-5)