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Which term of the ap 3,15,27,39,........will …

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Which term of the ap 3,15,27,39,........will be 120 more than its 21st term ?

Posted by Manvi . 4 years, 8 months ago

CBSE > Class 10 > Mathematics

2 answers

Yogita Ingle 4 years, 8 months ago

Given sequence is 3,15,27,39,...

The first term is a=3

The common difference is d=15−3=12

we know that the nth term of the arithmetic progression is given by a+(n−1)d

Let the mth term be 120 more than the 21st term

Therefore, 120+mthterm=21stterm

⟹120+a+(m−1)d=a+(21−1)d

⟹120+(m−1)12=(20)12

⟹12m−12=240+120

⟹12m=360+12

⟹12m=372

⟹m= 372/12 =31

0Thank You

Aachal Satwan 4 years, 8 months ago

a = 3, d=12 a21 = a + (n-1)d = 3 + (21-1)12 = 243 243 + 120 = 363 an = 363 a + (n-1)d = 363 3 + (n-1)12 = 363 12n = 363 - 3 + 12 n = 372 / 12 ∴n = 31st term

0Thank You

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