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Binomial Theorem question

2025 · 8 Apr · Shift 2 · Q46
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  5. /2025 · 8 Apr · Shift 2 · Q46

Binomial Theorem question

2025 · 8 Apr · Shift 2 · Q46

JEE MainMathematicsBinomial TheoremNumerical+4 / −1
The product of the last two digits of (1919)1919(1919)^{1919}(1919)1919 is
Numerical answer
View written solutionFree

Correct answer: 63

  1. We need the last two digits of 191919191919^{1919}19191919, i.e. the value modulo 100100100.

  2. Reduce the base modulo 100100100: 1919≡19(mod100)1919 \equiv 19 \pmod{100}1919≡19(mod100) So we need 191919(mod100).19^{1919} \pmod{100}.191919(mod100).

  3. Find a pattern in powers of 191919 modulo 100100100:

191=19≡19(mod100)19^1 = 19 \equiv 19 \pmod{100}191=19≡19(mod100) 192=361≡61(mod100)19^2 = 361 \equiv 61 \pmod{100}192=361≡61(mod100) 193=19⋅61=1159≡59(mod100)19^3 = 19\cdot 61 = 1159 \equiv 59 \pmod{100}193=19⋅61=1159≡59(mod100) 194=19⋅59=1121≡21(mod100)19^4 = 19\cdot 59 = 1121 \equiv 21 \pmod{100}194=19⋅59=1121≡21(mod100) 195=19⋅21=399≡99(mod100)19^5 = 19\cdot 21 = 399 \equiv 99 \pmod{100}195=19⋅21=399≡99(mod100) 196=19⋅99=1881≡81(mod100)19^6 = 19\cdot 99 = 1881 \equiv 81 \pmod{100}196=19⋅99=1881≡81(mod100) 197=19⋅81=1539≡39(mod100)19^7 = 19\cdot 81 = 1539 \equiv 39 \pmod{100}197=19⋅81=1539≡39(mod100) 198=19⋅39=741≡41(mod100)19^8 = 19\cdot 39 = 741 \equiv 41 \pmod{100}198=19⋅39=741≡41(mod100) 199=19⋅41=779≡79(mod100)19^9 = 19\cdot 41 = 779 \equiv 79 \pmod{100}199=19⋅41=779≡79(mod100) 1910=19⋅79=1501≡1(mod100)19^{10} = 19\cdot 79 = 1501 \equiv 1 \pmod{100}1910=19⋅79=1501≡1(mod100)

Thus the powers repeat with period 101010: 1910≡1(mod100).19^{10} \equiv 1 \pmod{100}.1910≡1(mod100).

  1. Reduce the exponent modulo 101010: 1919≡9(mod10)1919 \equiv 9 \pmod{10}1919≡9(mod10) Hence 191919≡199(mod100).19^{1919} \equiv 19^9 \pmod{100}.191919≡199(mod100). From above, 199≡79(mod100).19^9 \equiv 79 \pmod{100}.199≡79(mod100).

So the last two digits are 797979.

  1. Their product is 7×9=63.7\times 9 = 63.7×9=63.

Therefore, the required integer is 636363.

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