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59. an = a1 + (n-1) d

So an – a1 = (n-1) d

Best now to put brackets round Left side

( an – a1) = (n-1) d

Divide both sides by d to isolate the n part

( an – a1)/ d = (n-1)

So n -1 =( an – a1)/ d

Add 1 to both sides

n = ( an – a1)/ d + 1 answer

60 an = a1 + (n-1) d

Divide both sides by (n-1) to isolate the d part

( an – a1)/ (n-1) = d

answer to 60 can also be written

d =( an – a1)/ (n-1)

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