Question
Use the direct method to evaluate : $(3b−1) (3b+1)$
✓
Answer
Note : $(a+b) (a−b)$
$= a^2 − b^2(3b−1) (3b+1)$
$= (3b)^2− (1)^2$
$= 9b^2 − 1$
$= a^2 − b^2(3b−1) (3b+1)$
$= (3b)^2− (1)^2$
$= 9b^2 − 1$
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(a) 15 days$\quad$(b) 20 days$\quad$(c) 45 days$\quad$(d) 90 days
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→1. In how many days can A and B together build the wall ?
(a) 15 days$\quad$(b) 20 days$\quad$(c) 45 days$\quad$(d) 90 days
2. In how many days will the wall be built, if A builds it and B breaks it simultaneously?
(a) 45 days$\quad$(b) 60 days$\quad$(c) 75 days$\quad$(d) 90 days
3. In how many days will the wall be built, if all the three labourers $A , B$ and C are employed to build it?
(a) 10 days$\quad$(b) 12 days$\quad$(c) 15 days$\quad$(d) 20 days
4. How many days will it take to build the wall, if A and B work together to build it while C breaks it simultaneously?
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(c) 120 days$\quad$(d) The wall will never be built
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