A rectangle initially has dimensions 6 cm by 7 cm. All sides begin increasing in length at a rate of 4 cm/s. At what rate is the area of the rectangle increasing after 22 s ?

You can make a A. P. of sides at each second;

for 1st side, 6,10,14 ...

here, a = 6, d=4, n=22, an = ?

an = a + (n-1) d

an=6 + (22-1) 4

an = 6+84

an = 90

For 2nd side, 7,11,15 ...

Here, a=7, d=4, n=22, an=?

an = 7 + (21) 4

an=91

dimensions after 22 sec = 90cm * 91 cm

Area would be equal to 8190 cm²