Question

the 5th and 10th term of arithmetic sequence are 40 and 20 respectively . calculate its 15th term​

Answer 1

Step-by-step explanation:

here,

t5=40, t10=20 _( given)

:. tn= a+(n-1)d

:. t5= a+(5-1)d

:. t5= a+4d

But t5=40 is already given here

:. 40= a+4d

:. a+4d=40 ........(1)

:. tn= a+(n-1)d

:. t10= a+(10-1)d

:. t10= a+9d

But t10=20 is already given here

:. 20=a+9d

:. a+9d=20.....(2)

substracted eqn 1 and 2

:. a+4d=40 -(a+9d=20)

:. a+4d=40-a-9d=-20

:. -5d=20

:.d= -4

place d=-4 in eqn 1st

:. a+4×-4=40

:. a-16=40

:. a=56

:. tn=a+(n-1)d

:. t15= 56+(15-1) × -4

:. t15= 56+14×-4

:. t15=56-56

:. t15=0

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