If a= 5^99 x 2^98 and b= 5^72 x 4^38 show ab has 172 digits when written as a ordinary number This is a proof question. Please show step by step.

Step-by-step explanation:

ab

= (5⁹⁹ * 2⁹⁸) * (5⁷² * 4³⁸)

= 2⁹⁸ * 2⁷⁶ * 5⁹⁹ * 5⁷²

= 2¹⁷⁴ * 5¹⁷¹

= 2³ * (2¹⁷¹ * 5¹⁷¹).

The 2¹⁷¹ * 5¹⁷¹ will give 171 ending zeroes as 2 * 5 = 10.

And there is a 8 infront as 2³ = 8.

Hence altogether there are 171 + 1 = 172 digits.

the 4th choice is the correct one

step-by-step explanation: