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3 years ago

13

Seven times the difference of 9 and a number amounts to 84. Find the number.

1 answer:

Stels [109]3 years ago

4 0

7 * (n-9) = 84
7n - 63 = 84
7n = 147
/n /n
n = 21

Therefore the number is 21.

Proof:

7 * (21-9) = 84
7 * 12 = 84
84 = 84

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Given:

The sum of two terms of GP is 6 and that of first four terms is $\dfrac{15}{2}.$

To find:

The sum of first six terms.

Solution:

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Sum of first n terms of a GP is

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$S_2=\dfrac{a(1-r^2)}{1-r}$

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$\pm \sqrt{\dfrac{1}{4}}=r$

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