Class 10 Math All topics Arithmetic Progressions

If $t_{n}$ be the $n_{th}$ term of the A.P. $−9,−14,−19,....,$ what is the value of $t_{30}−t_{20}$?

(a)

$−35$

(b)

$−50$

(c)

$−55$

(d)

$−65$

Correct answer: (b)

Solution: Given series: -9, -14, -19, ....

$a=−9$

$d=(−14)−(−9)=−5$

$t_{30}=(−9)+(30−1)(−5)=−154$

$t_{20}=(−9)+(20−1)(−5)=−104$

$∴t_{30}−t_{20}=(−154)−(−104)=−50$ (Ans $→$ B)

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introduction to trigonometry

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