If a = 4011 and b = 3989, then value of ab = ?

A. 15999879

B. 15899879

C. 15989979

D. 15998879

Answer: Option A

Solution(By Examveda Team)

a = 4011
b = 3989
a × b = 4011 × 3989
         = 15999879

Shortcut method :
a = 4011 ⇒ (4000 + 11)
b = 3989 ⇒ (4000 - 11)
[(a - b) (a + b) = a² - b²]
According to the question,
a × b = (4000 + 11) (4000 - 11)
        = 4000² -11²
        = 16000000 - 121
        = 15999879