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Binomial Theorem question

2023 · 1 Feb · Shift 1 · Q38
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  5. /2023 · 1 Feb · Shift 1 · Q38

Binomial Theorem question

2023 · 1 Feb · Shift 1 · Q38

JEE MainMathematicsBinomial TheoremNumerical+4 / −1
The remainder, when 19200+2320019^{200}+23^{200}19200+23200 is divided by 49 , is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 29

  1. We need the remainder of 19200+2320019^{200}+23^{200}19200+23200 when divided by 494949.

So we work modulo 494949.

  1. Reduce the bases modulo 494949: 19≡19(mod49),23≡23(mod49).19 \equiv 19 \pmod{49}, \qquad 23 \equiv 23 \pmod{49}.19≡19(mod49),23≡23(mod49). A better observation is: 19=−30,23=−26(mod49)19= -30, \qquad 23=-26 \pmod{49}19=−30,23=−26(mod49) but an even simpler route is to use powers.

  2. Compute smaller powers modulo 494949.

For 191919: 192=361≡361−343=18(mod49).19^2=361 \equiv 361-343=18 \pmod{49}.192=361≡361−343=18(mod49). Then 194≡182=324≡324−294=30(mod49),19^4 \equiv 18^2=324 \equiv 324-294=30 \pmod{49},194≡182=324≡324−294=30(mod49), 198≡302=900≡900−882=18(mod49).19^8 \equiv 30^2=900 \equiv 900-882=18 \pmod{49}.198≡302=900≡900−882=18(mod49). So powers are cycling. In fact, 192≡18(mod49),196=192⋅194≡18⋅30=540≡1(mod49).19^2 \equiv 18 \pmod{49}, \qquad 19^6=19^2\cdot 19^4 \equiv 18\cdot 30=540 \equiv 1 \pmod{49}.192≡18(mod49),196=192⋅194≡18⋅30=540≡1(mod49). Thus, 196≡1(mod49).19^6 \equiv 1 \pmod{49}.196≡1(mod49). Hence 19200=196⋅33+2=(196)33⋅192≡133⋅18=18(mod49).19^{200}=19^{6\cdot 33+2}=(19^6)^{33}\cdot 19^2 \equiv 1^{33}\cdot 18=18 \pmod{49}.19200=196⋅33+2=(196)33⋅192≡133⋅18=18(mod49).

  1. For 232323: 232=529≡529−490=39(mod49).23^2=529 \equiv 529-490=39 \pmod{49}.232=529≡529−490=39(mod49). Then 234≡392=1521≡1521−1470=51≡2(mod49).23^4 \equiv 39^2=1521 \equiv 1521-1470=51 \equiv 2 \pmod{49}.234≡392=1521≡1521−1470=51≡2(mod49). Also, 236=232⋅234≡39⋅2=78≡29(mod49),23^6=23^2\cdot 23^4 \equiv 39\cdot 2=78 \equiv 29 \pmod{49},236=232⋅234≡39⋅2=78≡29(mod49), not yet 111. So let us continue more carefully.

A better approach: since 232≡39(mod49),23^2 \equiv 39 \pmod{49},232≡39(mod49), 233≡23⋅39=897≡897−882=15(mod49),23^3 \equiv 23\cdot 39=897 \equiv 897-882=15 \pmod{49},233≡23⋅39=897≡897−882=15(mod49), 236≡152=225≡225−196=29(mod49),23^6 \equiv 15^2=225 \equiv 225-196=29 \pmod{49},236≡152=225≡225−196=29(mod49), 2312≡292=841≡841−833=8(mod49).23^{12} \equiv 29^2=841 \equiv 841-833=8 \pmod{49}.2312≡292=841≡841−833=8(mod49). This is not simplifying quickly.

So instead use the fact that φ(49)=42\varphi(49)=42φ(49)=42, and since gcd⁡(23,49)=1\gcd(23,49)=1gcd(23,49)=1, 2342≡1(mod49).23^{42}\equiv 1\pmod{49}.2342≡1(mod49). Now 200=42⋅4+32,200=42\cdot 4+32,200=42⋅4+32, so 23200≡2332(mod49).23^{200}\equiv 23^{32}\pmod{49}.23200≡2332(mod49). Let us compute by repeated squaring: 232≡39,23^2\equiv 39,232≡39, 234≡392=1521≡2,23^4\equiv 39^2=1521\equiv 2,234≡392=1521≡2, 238≡22=4,23^8\equiv 2^2=4,238≡22=4, 2316≡42=16,23^{16}\equiv 4^2=16,2316≡42=16, 2332≡162=256≡256−245=11(mod49).23^{32}\equiv 16^2=256\equiv 256-245=11 \pmod{49}.2332≡162=256≡256−245=11(mod49). Therefore, 23200≡11(mod49).23^{200}\equiv 11 \pmod{49}.23200≡11(mod49).

  1. Add the two remainders: 19200+23200≡18+11=29(mod49).19^{200}+23^{200} \equiv 18+11=29 \pmod{49}.19200+23200≡18+11=29(mod49).

Therefore, the remainder is 29.\boxed{29}.29​.

  1. Comparison with stored answer: Stored correct answer = 292929. Our derived answer also = 292929. So they agree.
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