Question

. Find the A.P. whose 10th term is 5 and 18th term is 77.

Answer 1

hey mate hope it helps you...

Let a be the first term and d be the common difference

» a10= a + 9d = 5

» a 18 = a + 17d = 77

Solving it we get,

8d = 72

d = 9

a + 81 = 5

a = -76

Therefore the AP is -76 , -67 , -58 ......

Answer 2

Let's

the first term be x.

the common difference be d.

1st condition :

t10=5

tn=a+(n-1)d

t10=a+(10-1)d

a+9d=5 ...(i)

2nd condition :

t18=77

tn=a+(n-1)d

t18=a+(18-1)d

a+17d=77 ...(ii)

equation (ii) - (i)

ans≈

d=9

a=-76

t1=a=-76

d=9

t2=a+d

=-76+9

t2=-67

t3=t2+d

=-67+9

t3=-58

t4=t3+d

=-58+9

t4=-49

It is a AP.. -76 , -67 , -58 , -49............

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