N persons stand on the circumference of a circle at distinct points. Each possible pair of persons, not standing next to each other, sings a two-minute song one pair after the other. If the total time taken for singing is 28 minutes, what is N?

Simplify $\frac{81x^2-49y^2}{9x+7y}$

If $a+b+c=13$ and $ab+bc+ca=54$, then $a^3+b^3+c^3-3abc$ is equal to:

What is the simplified value of $\frac{(2.7{)}^3+(1.3{)}^3}{(2.7{)}^2-2.7\times 1.3+(1.3{)}^2}?$

Write the following statement in the form of equation. If you take 9 from 5 times x, then you get 85-

What is the simplified value of: $\frac{{\sin }^3 {21}^{\circ }+{\cos }^3 {19}^{\circ }}{\sin {21}^{\circ }+\cos {19}^{\circ }}+{\sin }^2 {69}^{\circ }+{\cos }^2 {71}^{\circ }+\frac{1}{\sec {69}^{\circ }\mathrm{cosec} {71}^{\circ }}$ is

Simplify:

${\left(\mathrm{a}+\mathrm{b}-\mathrm{c}+\mathrm{d}\right)}^2-{\left(\mathrm{a}-\mathrm{b}+\mathrm{c}-\mathrm{d}\right)}^2=?$

Expand : ${\left(\mathrm{W}-9\right)}^2$

1. $\left({\mathrm{W}}^2-9\mathrm{W}+81\right)$

2. $\left({\mathrm{w}}^2-9\mathrm{W}+18\right)$

3. $\left({\mathrm{W}}^2-18\mathrm{W}+81\right)$

4. $\left({\mathrm{W}}^2-18\mathrm{W}-81\right)$