book Maths Puzzle Numerical Ability

How do i find the nth rule when a3=15 and a11=5 (the 3 & 11 are subscripts)

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book Other Question

a line passes through a(1,-3) and b(2,7)
write the equation of a line perpendicular to a. how would you go about solving this problem? can you give me the formula to work out the hypotenuse when I only know 1 length & the three angles of the triangle